It's possible most, if not all, Republicans are just so stupid, lazy and greedy that they believe riding Trump's coattails is the path to fame and fortune.
OTOH, there's the Proud Boy Theory:
The Republicans don't want Trump around much less running for office or influencing any elections under the Republican banner. An impeachment stops Trump from running for office, but not from influencing Republican elections. Think Greene times a dozen.
Trump is a modern Caligula: A madman with power and
the chants of "Destroy the GOP" still ring loudly behind closed doors at Republican meetings.
Militias have been around since the American Revolution. Their function has evolved several times and their motives vary. Some are simply like drug-running motorcycle gangs. Some are more like book clubs with guns and periodic meetings at a rifle range.
Some, like the Proud Boys and Boogaloo Bois, are well trained, well motivated and include many vets with military training and discipline. This, of course, makes them very dangerous if they choose to attack government facilities and elected officials, their primary focus of angst. Think Timothy McVeigh with 20 clones in each city or state all grouped together. Normally they trained and complained but did little to harm others or break the law...until Trump came along.
How Trump knew about these assholes and tapped into them remains a bit of mystery, but there's no doubt he was, along with Miller and probably Junior. Trump praised them, focused them, motivated them and, after a few practice rallies, pointed them at the Capitol to kick off a civil war by taking over the US Capitol, killing most it contained and assassinating his own VP. A classic move in a Hollywood movie or a wargame. Obviously it failed miserably. Not just a near-miss failure, but the utter dejected failure of decent people who wake up from a blackout drunk and find they've committed a serious crime.
That failure is why many of these groups are falling apart, especially as thousands are arrested, even non-militia, and strive for plea deals in their sentencing. However, the hardcore center of the Proud Boys and Boogaloo Boise remain very proud, very dangerous and, now thanks to Trump, betrayed soldiers facing serious prison time because of Trump. Trump misled them in the attack and he betrayed them by saving his own ass rather than pardon them. That's the equivalent of an officer of a military unit running like a coward in the middle of the attack. It's a very serious betrayal of honor.
Now, during the impeachment, Trump is throwing salt in the wound by further betraying the very militias who were the backbone of his support. He's now labeling his own supporters, the men and women who put their lives on the line for Trump, as "criminal insurrectionists" and "lawless actors".
What are they to do? Go to prison for 20-30 years? Probably die there? Commit seppuku? Or go out man-style as a kamikaze? In that situation, I'd take Door #3. I've been around military people all of my life and, given I was stupid enough to get involved in the mess in the first place, it's a no brainer decision.
After all, these guys were prepared to line up most of Congress and the VP against a Congressional wall. They were prepared to kill and many, thanks to our own nation, were experienced killers. So who will be their final target? Another government building somewhere? Assassinate a couple of Democrats? Maybe, but it's a lame way to go.
Adding up all the elements, the best way to go out is to take out the person who organized all of this. End their lives putting a very large, very loud coda at the end of "the Trump Years".
My theory is that the Republicans know this and are simply letting it happen by doing nothing...of which they are very good at doing. If they upheld their oaths and convicted Trump, that's punishment. Some of the kamikaze boys might lose heart because Trump was punished for his actions. No matter how light, justice was done.
A better solution for the Republicans is to do nothing, let Trump get away scott-free and let the kamikaze boys solve both the militia problem and the Trump problem in a single event.
Just a theory. Time will tell if it's the correct theory.