It's a logical fallacy by most people to equate everything to one art or another when talking about MMA. The better question is, why hasn't his kickboxing evolved?
He's literally never going to have the level of wrestling of the guys you mentioned, but he doesn't need it - they aren't competing in wrestling. What he does need however is better footwork and awareness concerning the cage. That's not me saying he shouldn't train his wrestling, of course he should, but he needs to work on things that allow his best attribute to shine, which is his kickboxing.
If he had better footwork, managed distance better and overall maintained a better range between his opponent and the cage, you would see these wrestlers having to shoot from much further out, which would lead to less success on their part.
It reminds me of when JDS lost to Cain and everyone was clamoring about his wrestling even though he defended something like twenty plus takedowns and managed to get up every time. Honestly, for not being a grappler, his wrestling is great, it was his footwork that kept getting him in positions where he had to constantly defend TDs and his poor boxing defense that lead to him losing.