When you say "for one reason or another", who are you referring to? Sure we all wanted to see him fight Wand, Shogun, avenge his first loss to Rampage, and of course beat the piss out of Arona because he was so ****ing boring. But fighting anyone in PRIDE wasn't an option after winning the belt and I can't think of any legit contenders who weren't in the UFC or PRIDE at the time.
EDIT: Machida is the only one that comes to mind.
I'm referring to the fact that there are others out there Liddell didn't get the opportunity to fight, at the time where it made sense to fight them. The reasons aren't the same with Rousey, but I feel the same situation is happening with Cyborg not being in the organization, and Holm not being ready until now. It's kind of a poor example and hard to draw a parallel to anything. Maybe a more appropriate example would be Royce/Sakuraba.