OK, so the question is why? The proximate cause is that they were told, baselessly, from all sides of their media bubble, that the election was stolen. Many of the rioters also were followers of a bizarre conspiracy theory/alternate reality game that tells them that this is all part of a giant master plan.

The media and political environment has not been serving its function of properly informing citizens so that they can make informed voting choices, and so you have people trying to overthrow the government based on lies.

Now, not everything people believed in the pre-internet media environment was factual either, but there was rough agreement as to consensus reality. Even in the last contested election, the 2000 one, people roughly agreed on the basic facts (butterfly ballots, hanging chads, etc), even if they didn't agree on what should be done about it.

I don't have a solution for this problem, but it is a very serious one.