Why is the positive mitzvah to eat matza only for the first day of Peach?
If the psukim either state to eat for 7 days in Shmot or 6 days in Devarim, why is it a postive obligation to eat it only the first day?
The classic answer to your question is a Rashi in Shemos 12:15
For seven days you shall eat unleavened cakes-: But elsewhere it says: “For six days you shall eat unleavened cakes” (Deut. 16:8). This teaches us regarding the seventh day of Passover, that it is not obligatory to eat matzah, as long as one does not eat chametz. How do we know that the first six days are also optional concerning eating matzah?
This is a principle in interpreting the Torah: Anything that was included in a generalization in the Torah and was excluded from that generalization in the Torah to teach something it was not excluded to teach only about itself, but it was excluded to teach about the entire generalization.
In this case it means that just as on the seventh day eating matzah is optional, so is it optional in the first six days. I might think that on the first night it is also optional. Therefore, Scripture states: “in the evening, you shall eat unleavened cakes” (Exod. 12:18). The text established it as an obligation. — from Mechilta
However, (questioning the premise of your question) see this Yeshivat Har Etzion shiur. Here is the summary:
We have thus learned that, according to one view, there is an obligation to eat matza all week (Penei Yehoshua's understanding of Rav Shimon), whereas another opinion maintains that although there is no obligation, one fulfills a mitzva by eating matza all week (Chizkuni, Vilna Gaon). This latter opinion was disputed by Me'iri and others. Lastly, we saw that the Maharal felt that although one fulfills the mitzva by eating a ke-zayit on the first night, all matza eaten that night is also a fulfillment of the mitzva.