The eastern woodlands vs the oak-hickory/ prairie parts of indiana

Most of Indiana was composed of the eastern deciduous forest, a rich and diverse area composed of species like oak, sugar maple, ash, tulip tree, sassafras, dogwood, redbud, and tupelo.

However, indiana was at the western fringe of this area. Soon after crossing into illinois , it transitioned to the oak-hickory and prairie biome. Praries made up large areas and where forest do occur, it is noticeably different than the eastern deciduous forest. The trees there are predominantly oak and hickory with maybe some ash and elm. You lose alot of the eastern species like sugar maple, tulip tree, dogwood, tupelo, etc . Less overall species. Some sugar maple forest do exist in illinois but only along river valleys and steep slopes. Much rarer. This oak hickory/ prairie biome also extended into extreme western central Indiana and a larger chunk of northwest Indiana.

Ive always found this interesting especially in northwest Indiana because the transition is so sharp. You can walk around a forest by Indiana dunes and it's almost full on eastern woodlands with sugar maples and tulip trees and dogwoods, but drive twenty mins west into lake county and it's full on oak hickory. Nary a sugar maple or dogwood or sassafras to be seen. However, I've noticed some eastern species like sassafras continue along the old sand ridges. I've seen a few pockets of sassafras and tupelo just barely past the illinois state line.

I've always wondered why this is? The precipitation between Indiana and Illinois doesn't seem that different.