I'm a Special and I'm not riot trained so take my comments with a pinch of salt, but from talking to officers who are, it's all about officer safety. They'll go in when they know they have a numbers advantage. The Police's priority is to stop disorder but not at the expense of getting officers injured. At the moment the Met don't have enough officers to deal with all the rioters, but many officers are getting drafted in from other counties on mutual aid. This should change the game a bit.
Yes. Let's enjoy seeing riots in other counties tonight instead with not enough police to stop them there.