Because convictons for thease sorts of crime are life and liberty ending. If any charges warrent a trial by jury then thease charges do because they are charges that cause the accuser to quite likely go to prisson and, more impornantly, they are cahrges which cause you to be labeled, by socierty, as a "propper criminal" (unlike, for example, a speeding fine).
But basically I feel that if I were in this position, I'd want a jury trial (or at least a right to a jury trial)
I sat in on a case today (just in the public gallary), an assault / battery, where the case come down to one person's word agaisnt a couple of others.
I would have found the defendent not guilty, and the DJ found him guilty. I'm not saying that this was a miscarridge of justice (although I belive it might have been) and I don't know how a jury of 12 people would have decided the case but I think the defendent should have been given that chance.
Furthermore I know that his perception was that he wanted a jury trial (from overheard snippets of conversation he had with his solicitor). He clearly did not feel that jusitice was being done and he would have rather had a jury trial.