Another factor to consider is sunshine.  The farther east you go the more moisture in the atmosphere being pumped up from the Gulf, more pollution, etc.  There is just more sunlight per square foot west of teh Mississippi than there is east of it.

And as mentioned the depth of the profile is significantly different.  In FFA (yes they had it when I was in school, it was Future FARMERS of America back then) I went to a soils judging contest held about 50 miles north of my school.  I looked down in this hole a backhoe had dug and it didn't look anything like what I was used to.  No yellow clay, no red gravel, no blue muck, just very black stuff that belonged in a flower pot.

Oh, and a surveryor told me one time as the surveyors laying out the sections came west their chains would wear.  If you put 42 or 43 acres in a quarter section it tends to yield better.