Yw fan: you ask a very good and fundamental question. (yes tcg I did wonder where this thread would lead when I opened it). Probably the reason is along the lines that bread can either be allowed to rise or not be allowed to rise. Its either leavened or unleavened, either chametz or matza. We must commemorate the manner in which hashem brought us out of mitzraim in haste, where we show that yeshuas hashem is always kheref ayin, and we are instructed to do this by remembering their bread which was not given time to rise before they were evicted from mitzraim.
My 2 long time questions are: 1. The pasuk says they were driven out of mitzraim, implying as if they were woken up unexpectedly in the night and told to leave immediately. But we know this was not the case. Paroh asked them to leave by chatzos halayla immediately after makkas bichoros when he came looking for Moshe but Moshe said no we will not leave in the middle of the night but in the day and they waited until morning before leaving. So why did they not begin baking their bread on time; they chose their time of leaving. They were not suddenly chased out?
Question 2. The pasuk says matza zu sheanu ochlim al shum mah… Because the dough of our fathers did not have time to become leavened before hashem revealed himself to them and redeemed them. QUESTION: Had they not been ‘chased out’ and had their dough been allowed to rise, would we still have a mitzvas matzo and issur chametz? Is this the entire and only reason for chametz and matzo? Would we not have had this mitzva anyway had they been allowed to leave without pressure?
(I have a mehalach on these questions but post them to see what the learned coffeeroom can come up with).