It doesn't make scientific sense, but what I heard on a radio news report is that the BP folks believe using the well in any way will create major public relations problems. My understanding is that if they do access that field it will be through a completely new well.
It sounds stupid to me - drilling a new well is expensive, and there are risks - but it's their money to waste as they wish.
Maybe I'm just stupid about oil drilling but I don't see why the relief well effort can't just be another tap into the same oil supply that we know is there. If they pump mud and cement successfully into the blown out plumbing, why drill another hole to pump in more mud and cement? I could understand if the relief well was to take pressure off the damaged well to help plug it up but it's already plugged now....or as good as plugged anyway. Why not just drill a proper new well and start bringing up the crude? We already know where it is.