The motive is in question. Whether it was intentional or an accident. If they say it was intentional now and then later it's somehow proven the guy was spook/scared so he hit the gas to get away then he could sue the PD and city.
Trying to be both careful and thorough is my guess. They have the guy, know who he is, and he isn't going anywhere so time is on their side.
Plus they’d likely want to check the guy out, search his house etc. If he’s affiliated with any groups that might be involved, the cops won’t want to do anything that might tip them off and possibly trigger the destruction of evidence before the cops get to it.
Cops have a responsibility to execute their investigation the best they can, and among other things that means wringing any possible advantage they can get out of the suspect’s name being kept out of the media. If that means they don’t immediately feed the media frenzy every bit of information they have, then so be it.