I've noticed high level wrestlers in the UFC going from completely dominant in the early days of the UFC & then less so recently. A part of that is fighters evolving on the receiving end of TD's, recognizing the threat & becoming skilled at TDD & escaping from the bottom.
Still, these wrestlers often fail to even try their wrestling, which is a falacy, since even the attempt of TD's makes the receiver have to change his stance & gives him more to worry about on the feet & so it offers an advantage on the feet if you have that threat.
The best I can make of it, is that the cardio expenditure for wrestling is much higher, & it gets worse if the receiver of a TD can avoid them or escape. The 2nd thing is how wrestlers like Cain & DC basically had to lighten their wrestling in fights due to the wear & tear on their body. (it's especially hard on the knees & neck.)
If you got a wrestler that is in top physical form, they've honed that skill from childhood (GSP being an exception) & so by the time they make the UFC... their body has already taken what could amount to long term permanent physical conditions. As these conditions take hold in them as more & more time goes by, they simply can't do it the way they used to. (kind of like Aldo waz a terror with leg kicks before he reckognized the long term damage & so had to quit throwing them as often.)