They don't actually have that similar of geography. Mississippi lies completely in the Atlantic Coastal Plain (the area known for plantations across the South), while only a little over half of Alabama is. The end of the Appalachians occurs in Alabama, just south of Birmingham.

Since it's the end, a lot of the subdivisions of the Appalachians, like the Ridge & Valley, uniquely hit the coastal plain only in Alabama. This creates some rare soil mixes at those points.

On top of that, Alabama's rivers quickly spread out like a fan. So there are many different rivers that cross the fall line (the meeting of the coastal plain with the Appalachians), like the Tennessee River, the Black Warrior River, the Cahaba River, the Coosa River and the Tallapoosa. The fall line tends to isolate aquatic ecosystems above the line, so Alabama ends up with five or six uniquely isolated ecosystems in the northern half of the state.

Then there's the Mobile Delta in the southern part of Alabama. Like any other major swampy area, it has all the pieces needed for massive biodiversity (lots of water, mild winters, longer daylight, etc).

Yet, it's not just that. During the last Ice Age (which, in relative terms, wasn't that long ago), Alabama wasn't covered in glaciers. So it really wasn't a mass extinction event there. In fact, a lot of the wildlife that would be associated with anything south of taiga got pushed down into Alabama. And for other than things far more inclined to cooler temperatures like elk, most of that wildlife stayed.

These things combined make Alabama the most biodiverse state east of the Mississippi, and not Florida like many would assume. Other nearby states have similar aspects, but not in the same degree. For example, Georgia has pretty much all the same things except the different Appalachia strata hitting the coastal plain.