I think I got this from the book about UC, that when the programme started there weren't enough universities to make a long enough series, so they were invited to enter as colleges. I've just been to check Wikipedia, which claims that they're limited to five per series each, which I didn't know about, and only makes it slightly better. I'd rather have more universities entering though, more interesting to watch. (Ooh, and the Wikipedia page has a small list of cheating Oxford teams! Cheating is a bit strong. But still.)