A few possibilities:

- Overcorrection: law enforcement had come under fire for using excessive force against protests and they erred on the other side.

- Conspiracy: the security state is firmly under the control of the pro-Trump faction, who (incorrectly) thought they could make Trump president for life once the Q Shaman took the Senate.

- Conspiracy: the security state is firmly under the control of the anti-Trump faction, who (correctly) thought that letting the protest go too far would be politically devastating for their enemies, and would provide the hook for passing new security laws.

- Incompetence: America can't protect its capitol for the same reason it can't distribute a vaccine, build railways or put a man on the moon.