I've always assumed that the LDS wealth/activity correlation was somehow a function of "commitment" levels. Basically, it takes a lot of commitment to be an active Mormon. Without generalizing too much, I think it's fair to say that many, if not most, of Americas uneducated poor lack the ability to commit to things and follow through, making Mormonism a bad fit. Much better fit for the typical MBA.

Lots of other faiths that might require lower commitment levels are good fits for America's uneducated poor.

What is still really remarkable though is that becoming educated does not, apparently, lead to large scale defection due to people becoming jaded of whatever. Seems to happen everywhere else, but to a lesser degree in Mormonism. Never been able to wrap my head around a reason for that.