Its easy to arm-chair quarterback this one. Left or right its easy to see malice here, but it is worth remembering that this was pretty much unprecedented.
The pentagon offered help but that was days before the riots and I don't know that they (the police) had the proper intelligence reporting to make a good decision there. *That is a point of concern, because the idea that we have intelligence but it isn't getting to the right people is troubling.* One of DHS's missions is to do that.
I believe there were 300+ NG, but they were being used for traffic. I know people were apparently concerned about the optics of Armed soldiers guarding our transition. I'm sure that sounded a lot more reasonable at the time.
According to the timeline, things turned bad pretty quickly. With the military chain of command what it is, it isn't necessarily easy to call in a bunch of troops. Especially since they aren't cops and aren't really expected to respond like that. They would have needed to be prepared and briefed, equipped, etc. Transportation would need to be arranged. What they should have done is accepted the help with an understanding to have them on standby. But again, that's easy to say in hindsight.
A lot of the responses are like that. I bet in the future they will drill this to death and always have a plan for how to respond to such threats and by the time (hopefully never) its needed again we will probably have relaxed our standards again.