Given the proliferation of published commentaries on the Haggada these days, I'm sure there are an abundance of answers to this question. The two most famous answers, though, come from earlier commentaries.
The first from Abarbanel, who asks this in connection to what "לחם עוני" means. What is "poor bread"? Answering that "poor" refers to the composition of the dough as well as it's quality of being filling in small amounts, Abarbanel writes that in "Ha Lachma Anya", we refer to matza by it's inherent qualities of paucity, while only later in the Haggada do we discuss what the matza represents, which is freedom and redemption.
The second answer comes from the Maharal, who's writings on a Haggada (although not initially composed as a commentary to the Haggada necessarily) were clearly meant as a response to Abarbanel's Haggada. His answer is basically that given by Chabad.org and quoted by @sharshi in his answer. Matza is called lechem oni because it is the poor man's bread. But poverty and freedom are not contraditory ideas; on the contrary. Poverty is essentially freedom in that one who is anchored by his wealth and possessions is not as free as one who is not. Thus matza's designation as לחם עוני is itself a representation of freedom and redemption.