Why isnt URF random pick?

They answered this in the FAQ. Basically, they agree that it would be better for strategic choice, but they felt really bad about implementing it. They wanted everyone to have a chance to play what they like. URF's return is, after all, fan service. A paraphrase of their analogy, "If you really, really wanted to see what Yorick in URF is like, you'd only have a 1/100 chance of ever seeing him".

Because some people bought champions that they actually don't like playing?

Why should it be random? And before you list off champions you see every match, compare your list with the other 50 lists out there and realize you've got different lists, so they can't be in every game.