I don't think political correctness has anything to do with it. The "historical preservation" crowd goes through phases, and the current teaching is that monuments in parks are bad things. They feel that preservation means that things should not be changed from the way they were at the time of their historical significance, so any markers should be simple things that can be moved or replaced or changed as the need arises. They don't believe in the "big granite monument" type of marker. When Fort DeRussy was turned over to the state, all of the "big granite monuments" had to be put up before the property was turned over, else they'd never have been allowed. Of course, now that they're there, this same axiom prevents them from being removed.
At any rate, you don't have to agree with this theory - I certainly don't - but that's how the latest crop of State Parks people think. It has nothing to do with political correctness.