Because she hasn't been convicted of anything yet.

A fictional scenario:

Your idiot friends decide to rough up someone. You try to talk them out of it, but follow them hoping you can try to keep them out of trouble. Then three of them pull out knives and decide to stab the guy to death while you watch on in horror. Later, everyone that was there is arrested and charged, including you (which is the right thing for the police to do). Should the press release your name as "vile murderess who stabbed a man to death" before you've been convicted of anything, or even tried?

It's almost certainly not what happened here. But it might be, and we won't know until the facts come out in the trial. There will be plenty of time to release names after the trial (assuming being a minor doesn't exempt her completely). She's not going anywhere.

In general, with a few exceptions (including for public figures, whose trials will be known by the public whether they're released or not), names shouldn't be actively publicized before trial. That is regardless of the crime in question (from jaywalking to murder). Police make mistakes, and the accused is innocent until proven guilty. The severity of the crime has nothing to do with whether they actually did it.