Its definitely because of rights. I distinctly remember how long it took them to get Cactus Jacks real theme song because the ownership of it changed so much it took them years to track down.
I do like his and zbyskos replacement themes though.
Yes. When they revealed a few years ago how they finally got cactus theme it is that wwe purchases the right to use those songs for years at a time. When it expires if wwe hasnt renewed then they lose the rights to use them.
Kinda on topic- You dont see Gangrel in the games because white wolf entertainment owns the name and character rights. Thq got him for wwe 13, but had to do it on their own and while he was on the original roster for 2k14 (according to Cory at the time) White wolf raised their asking price so much that it made it impossible to use him.