There was actual competition between promotions. Monopolies are not good for anyone except the monopolizing entity.
Wrestlers didn't have to destroy their bodies at such a quick rate in order to keep up with the insane standards that had been set for believable finishes (Jake Roberts' DDT and Macho Man's elbow drop used to be considered devastating moves that no one could conceivable overcome).
I'm convinced the first statement here is the answer. Ever since WCW died, WWF/E had felt no real competitive push. TNA has gotten bigger, but is still nowhere near real competition for WWE. As a result, they've been more afraid of doing something WRONG, than they are in favor of doing something new or different. And the result of that is stagnation.
The 2nd statement here(wrestlers destroy their bodies with high risk moves ) also has some merit. Not only is is bad for the health of the guys, but it numbs the audience to everything. Where do you go after a guy has attempted what looks to the audience like human sacrifice?
"I'll go through Handsome Jimmy like Ex-Lax through a widow woman." -Jerry "The King" Lawler