I was wondering about this also. Actually, one of my initial thoughts was they are going to probably wall off DC for the inauguration in a way that has never been seen before, but then I thought about routes getting out of the city and these criminals fleeing the city.
They certainly could have grounded flights going out and set up some check points along the major thoroughfares out of the city, but realistically they probably needed time to review the tapes and identify people, so would have to balance that with “inconveniencing” other people. On 9/11 we grounded planes, but there was a possibility of a threat of another plane hijack.
I think it wasn’t taken seriously enough. I watched some video of people who were part of the rioting and many of them seemed very naive and clueless how frightening a mob like that is, how much danger they put themselves in, and how insane it is to storm the US Capitol. This one young man had the blood of the woman shot in his hands, and what he was saying was like he was surprised and the government will shoot people who disagree. Like we are Cuba or North Korea I guess in his mind. I could look for that video. Not that I give a shit about how naive some of these people are. My only point is maybe officials didn’t see them as an immediate threat to the community.
Moreover, a lot of police were on Capitol grounds, and there are multiple ways out of the city as mentioned above, but still some efforts could have been made.
Side note: Regarding DC airports, Washington National Airport, now known as Ronald Reagan, has a curfew of 10:00pm unless it has changed since I lived there. That would be the closest airport, so it actually has no flight movement 10:00pm to 7:00am already, but there are two others in the area, one in Maryland and one in Virginia that are 24 hour airports. I wonder if Maryland or Virginia governors can close those airports? When COVID first hit big in March, New Yorkers were pouring into Florida and we Floridians were told our governor couldn’t stop it, that air travel was under federal control and the president wouldn’t stop the flights. I don’t know if that is for sure true, and I don’t know if that might be part of the reason.