YW fan…
(Firstly- I mistakenly called Shemot, Numbers. In English, it’s Exodus)
There are many reasons given.
The Torah itself just says…
12:39- “And they baked unleavened cakes of the dough which they brought forth out of Egypt, for it was not leavened; because they were thrust out of Egypt, and could not tarry, neither had they prepared for themselves any victual.”
Soooo… from a practical point of view, they HAD to eat matza because there was no time to wait for dough to rise to bake regular leavened bread. BUT… The eating of matza itself AND the total getting rid of chametz, are both mitzvot, – so there is a lot more to it than just the factor of lack of time. There are tons of commentaries on this subject that explain what chametz signifies, why it must be eliminated COMPLETELY, etc. and it is way to much to go into here.