There might be a good discussion on this subject somewhere in here.
The FBI and other intelligence agencies had monitored the rightwing chatter and had warned that Jan 6 activity was coordinated and potentially violent.
Washington D.C. is no stranger to demonstrations and partisan activations. The Proud Boys and other extremists had been pulling public stunts for months. The President of the United States had told them to stand ready during the Presidential Debate, presumably awaiting his command.
What exactly would happen on January 6? A full scale insurrection to invade the Capitol and stop the certification of Joe Biden? Or a louder than average circus of Trump supporters waving signs and saying stupid things to TV reporters? If you had to bet, which way would you go? And if you went with the most dangerous and unprecedented option, would a National Guard and all-out show of D.C. cops quell or inflame the mob? Had the National Guard stopped the protestors in their tracks, we would never have heard the end of the paranoid over-reaction by the anti-Trump D.C. forces.
Now consider that debate spread between all the law enforcement, political, and intelligence entities involved. Classic clusterf#&%.
It's kinda like 9/11. If you put together all the information between the FBI, the CIA, and the FAA, there were some very clear warnings about the potential attack. But these agencies often think they're in competition with each other. They don't like sharing information or credit. They don't communicate well. And when the Republican President is getting a briefing from a previous Democratic administration, they reject it out of hand.
In most cases I'll go with childish pride and risk avoidance over nefarious conspiracy.