I hear what you're saying, but I think what those examples combined prove is that there is no set-in-stone rhyme or reason as to how the UFC decides who gets the next title shot. They, loosely speaking, generally try to give it to whoever is "most deserving," but that's not always a clear, easily-quantifiable thing to define, so they also take into account what would make for an interesting fight that the fans would get excited for and buy tickets/PPV's for.
With that last point in mind, I have to think that the fans' interest level in seeing Randy fight Tim next is ranked somewhere around 10th, after CroCop, Nog, Arlovski, etc., etc., etc. Sylvia was never liked very much to begin with, and often put on pretty boring fights even when he was champ.
The UFC wants to make money.
Plus, Randy's fight with Sylvia wasn't even close. It would be one thing to consider a rematch pretty soon if it had been a pretty close fight, but it was basically a 5-round shutout.