Why did Israel have to eat unleavened bread when they left Egypt?
Israel ate unleavened bread because leaven in Scripture denotes evil or sin, and it had to be left out as they departed Egypt. The unleavened bread pictured them as a separated people, set apart unto God and no longer entangled in the paganism of their Egyptian masters.
Les explains that leaven in Scripture always speaks of evil, sin; it is yeast, and once put into bread dough it affects the whole lump. Since leaven denotes evil, it was to be left out of their bread dough as they made the exodus out of Egypt. The unleavened bread was a picture in type: Israel was now to be a separated people, separated unto God, no longer wrapped up in the paganism of their Egyptian masters. The passage also notes their sojourning in Egypt was 430 years.
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.