University Challenge would be better, in my view, if the bonus questions related to the starter question. But then, I'm just saying that because I'd have loved to hear what they might have been...
Sometimes they do, in the picture and music rounds for example. The difficulty occurs when nobody gets the starter right so to make that work you would have to have a set of bonus questions for each starter meaning a lot more bonus questions would be needed, many of which would never get used.