The Washington Post has a brilliant and thorough summary of the lead-up to January 6th. It goes into great detail about how law enforcement had a general sense that something awful was about to happen, but various institutional inefficiencies and biases worked against proper preparation for what transpired. You really should read the whole thing.
A few things stood out to me.
First, the descriptions of the Trump supporters usually included "though so-and-so had never really been active in politics before" or one who voted for Bush, then Nader, then Obama. These are not typical voters, and they represent a substantial section of Trump most fervid supporters. These atypically political people were the ones who were most likely to see Trump as some sort of messianic figure. Trump's non-traditional background and rhetoric were what appealed to them.
They are the ones who are both the most dangerous and the most likely to remove themselves from politics without Trump there to activate them.
Second, it has been apparent for years that the FBI and other law enforcement agencies are simply not willing to look hard at domestic terrorism. Reading the Post's account was like reading the 9/11 postmortems. In both cases, all the information was there, but no one had the authority or motivation to act on it.
Finally, there is no way January 6th happens without social media. The delusions and paranoia found an echo chamber that was not available to them two decades ago. Sure, there was the Brooks Brothers' riot from 2000, but that was a handful of GOP staffers bussed in to intimidate one ballot counting facility. What transpired throughout the fall and early winter of 2020-21 was both organic and carefully scripted. Online connections turned the lunatic ravings of Alex Jones and Rudy Giuliani into gospel truths. We see the fallout of this today with the bizarre anti-vax movement.
It is imperative that future elections are free and fair. At the moment, it seems likely that they will be, but Trumpists are trying to remove state-level officials. This could create dynamics that weren't in play last time. Last time, local Republican officials in Georgia and Arizona held firm. There is little hope that will play out the same way. As we approach future elections, the FBI and others cannot be hamstrung in preventing violence ahead of time.