Surely that was out of concern for their own safety (to retreat can be a valid strategy) and / or not willing to open fire on US civilians in such a symbolic place.
It's still a shame. I do wonder if there's something else they could have done. I don't know anything about how hard it is to stop a car.
The latter assumption is a serious mistake. Sure, the mob is not as well trained as professionals, but when you consider the number of vets with some military experience, the fairly large numbers of militia organizations which provide amateur levels of training in both virtual and real contexts, and the high availability of training materials (military manuals etc), it's smart to assume that any given mob will contain some people who know what they're doing and are able to direct others.
But DC has rather strict laws about firearms, so it was rational to rely on a swarming approach and then try to exploit the gains from that tactic. While the attempt to take over the capitol failed insofar as Congress reconvened to finish the process of counting electoral college votes, and many of the participants have been or are about to be arrested, the far right has scored an outstanding propaganda victory (complete with photogenic martyrs) at little real cost. I would say 2021 has got off to a spectacular start for them.
https://mobile.twitter.com/JoshuaPotash/status/1347277542588...
If your aim is to stage a putsch for real, you wouldn't be bothered by some gun laws, you would've gone all in.
But you make a good point; perhaps the plotters over-relied on shock value and ended up half-assing it, or perhaps there was no more plan other than stirring up disorder and hoping to capitalize on an emergency situation.
I'm pessimistic about the long-term implications; the far right gave up on the idea of a political solution some time ago and the less far right is now about ready to join them.
I expect (given the consequences of allowing them in and prior use of lethal force against demonstrations by Americans in America) the security personnel to be ready, willing, and able to use lethal force to keep armed protestors out, even if the doors were open.
I expect the doors to be strong enough to withstand anything short of an RPG.
I do not expect security personnel to desert their posts in their own capital city.