The problem is, they didn’t see it that way. These weren’t their enemies, these were their allies - Proud Boys, MAGA, “blue lives matter,” the kinds of folks they pal around with on the street, their common enemies against BLM and Antifa*, but also legislators and state attorney generals. Cops are, in general, incredibly pro-Trump, with many police unions actively supporting him. These were their people - quite literally, given the number of off-duty cops spotted among the rioters. Whatever was going on, the cops fundamentally weren’t viewing them as “protestors” but those with a common goal.
Basically there are two options here, neither of which are good for the cops - either someone on the command level decided, “let them do what they want, it won’t be any big deal” or were deliberately enabling the coup that was being publicly discussed beforehand (which is what foreign security officials believe). The first option requires them to have thrown standard procedures out the window, in an act of criminal negligence. The rioters were publicly discussing plans to storm the capitol building and execute legislators and Pence, so it’s not credible that the police were unaware of it. The most cursory standard intelligence gathering would have revealed that. So either a standard part of the process just didn’t happen, because the cops were so sympathetic to those involved, or they actively ignored it. This tends to suggest that somebody - not all the cops, but somebody at a significant level - was at least passively supporting a coup.
*The irony is that, in between this and the “Boogaloo” attacks in California, these right-wing groups that the police see as their allies have murdered a number of cops in the last year, while BLM and Antifa haven’t. Yet this doesn’t seem to have registered with law enforcement, at all.
