I'm not sure. The name usually comes out later in the day, doesn't it? Perhaps it's to do with police procedure? (Checking it's the right person, checking the accusations were against them and constitute a crime having been commited etc etc., making sure the paperwork is in order.)
As I've said before - a false accusation of something like this would be awful, but ,on the other hand, releasing a name can encourage others to come forward with information.
Perhaps - it would surely be better to wait until they could announce all the main details first though, especially if its only a matter of hours until that can happen.
Such accusations can ruin someones career, especially if their in the entertainment industry. I'd like to think they wouldn't arrest anyone without having built a pretty clear case against them or having some decent evidence, otherwise they'd just be asked in for questioning, which is voluntary(?).