One of the questions (examples?) in the Mah Nishtana is why on all other nights we eat leavened bread or matzah and tonight (Passover) we only eat matzah. However in the introduction to the seder we state "This is the bread of our affliction..." which would seem to explain why we are eating matzah. Since this introduction is said before the Mah Nishtana how do we understand the question regarding eating matzah?
I'm going to give a more simplistic answer, which is how I understand the haggada based on the commentary of Abarbanel.
When we say Ha Lachma Anya, we are not referring to the reason why we eat the matza. Lachma Anya is the Aramaic version of Lechem Oni, which is how matza is described in the Torah. But rather than translate that as "bread of our affliction", perhaps a better translation would be "meager bread", referring to the "poor" makeup of the matza and it's meager appearance. We do make a passing reference to the fact that our ancestors ate this bread in Egypt, but we make no mention of why they ate it in Egypt. True, they ate it with the Passover offering before they left, but why? The point of this paragraph is not to address this issue, so we don't get into it until later, when we mention R' Gamliel's statement.
One of the questions of the Mah Nishtana is about the matzah, but notice that we never specifically answer that question until much later. Avadim Hayinu doesn't seem to answer it at all. Truthfully, the point of Mah Nishtana is not to ask about the matzah and marror and other things specifically, but rather to ask about the following: Why do we do some things (like matzah and marror) that imply poverty and slavery, and then other things (like dipping and leaning) that imply freedom and wealth? Why are we being inconsistent? The answer to this is going to be that because we were freed on this night. We started off as slaves and became free men. So we so things to commemorate both. That's all that Avadim Hayinu is coming to say.
Now, this question comes from the perspective of someone who already knows a little about what's going on. We haven't taken out the marror, we've only dipped once, we made only a passing reference to matza. This is not an ignorant child asking. In fact, Abarbanel believes that this question is not necessarily designed for children. We are asking the question ourselves; the question-answer format is part of the traditional setup. It is only over time that the tradition became for the children to say the Mah Nishtana since it is their general roles to be asking questions on this night, just their questions weren't "canonized" into the haggada. So it became their "job" to recite the Mah Nishtana; but of course we would still recite it regardless if any children are present.
So, to answer your question, why would anyone ask the reason for eating matza when we've already said that matza is lachma anya? Well, first, because lachma anya has nothing to do with why we eat matza. And secondly, because the question of Mah Nishtana is not why we eat matza, but why we do things that imply slavery and poverty at all.
Firstly, it should be noted that the text of Mah Nishtana predates that of “Hei Lachma Anya”. Ma Nishtana was around in the time of the Beis Hamikdosh (see Pesachim 116a), whereas according to most opinions “Hei Lachma Anya” was established during the exile in Bavel, as is evident from the fact it was written in Aramaic (Machzor Vitri, Ravya Siman 525, Rashbatz in Yavin Shemu’a 49:3). Therefore, the question is really not on the Mah Nishtana, but rather on the placement of “Hei Lachma Anya” beforehand.
The Hagadah “Siach Yitzchok” (Hagr”i Milatzan, page 7 – quoted in HaSeder Haaruch vol. 2 pg. 124 footnote 6) asks this question, and suggests that due to the concern that the children might fall asleep before “Mah Nishtana”, and will miss out hearing the story of the Exodus from Egypt, they placed an abridged version of the story at the very beginning of the Seder, in “Hei Lachma Anya”. However, this is very difficult to fathom for several reasons: 1) Mah Nishtana is the very next paragraph, and we then immediately answer by saying “Avadim Hayinu”. If we were concerned the children might fall asleep, we could should have placed it at the begining of the Seder, not minutes before we answer the questions! 2) “Hei Lachma Anya” does not even mention that we were slaves in Egypt and came out, just the food we ate there. (I did not see the actual Hagada, just what was quoted in “Haseder Haaruch”, so it is possible he addresses these concerns).
In contrast, several Rishonim (Machzor Vitri, Rashbatz in Yavin Shemu’a 49:3) explain that “Hei Lachma Anya” was actually placed before Mah Nishtana in order to let the children know that we will only be eating Matza tonight, and they will be aroused to ask the Mah Nishtana. This too is hard to understand, as a general question can be asked on the rest of Mah Nishtana – how do the children know we are going to eat Maror, dip it in Charoses, or that we will only be eating leaning? If they deduce it from the contents of the Seder plate, why is Matza any different?
Others explain (HaSeder Haruch vol 3. Page 119) that paragraph only serves to answer why we just broke the Matza in half (it is “bread of affliction”, and we are like poor people who hide some of their food for later), but not the main question of why eat Matzah. However this does not fit with the fact that in most Haggadas this passage comes after the symbol for “Maggid” (see the Rambam’s Nusach Hahagah for example).
The question can be answered through a deeper understanding of what exactly “Hei Lachma Anya” means. There are several strong questions on this paragraph:
Based on the above questions, the Lubavitcher Rebbe (Likkutey Sichos vol. 17 pg. 80) concludes that it is clear that something very different is happening in this paragraph, and suggests the following novel and fascinating explanation:
This paragraph is actually a prelude to the story of the Exodus, and is pivotal in order to clarify a troubling problem that we are faced with as we embark in relating the story of the Exodus. As we recount the story of how Hashem took our ancestors out of Egypt, we are bothered by the question: What is this all worth if today we find ourselves in exile again? As we say later in the Hagada, “In every generation they rise against us to destroy us”. The Jews are dispersed across the world, hated by their neighbours and constantly persecuted. Sure, historically it’s a fascinating story, but practically speaking, what are we celebrating?
We therefore ask – could we possibly say (metaphorically speaking) that our ancestors tasted the “bread” of redemption? This is “bread of affliction”, as the redemption was short-lived and we are back in exile. Just look around and notice that there are starving people who cannot afford the basic needs of a Seder and need to be invited to other homes! We are slaves in exile, what is there to rejoice for tonight?
We answer this question with the closing words of the passage: “Next year we will be in the Land of Israel ... next year we will be free people.” To explain, one could ask the question: If Hashem is so perfect, how come He could not have made the redemption from Egypt a complete one, not followed by further suffering? The answer is of course He could, but He wanted it to come from us. Nonetheless, Hashem’s part in the redemption was complete and Yetzias Mitzrayim continues to provide us with the potential to fulfil our part. We are thus commanded to remember the Exodus every day of our life, for this is the Divine source we need to utilize to actualize the finale redemption.
So to answer the question: This paragraph does not contain an explanation for why we eat Matza - that will be addressed later in the Hagadah when we ask "Matza Zu She'anu Ochlim.." It's purpose is to explain the relevance of the celebrate of Pesach nowadays. It is therefore understandable why the child asks why we eat Matza in the "Ma Nishtana".