I believe it was was most likely for a multitude of reasons, not a single reason...
1. Protect them from their fanatical and now uncontrollable Marxist wing, BLM and antifa.
2. Protect them from a highly unlikely conservative assault
3. Make sure a HUGE Trump rally did not assemble on the mall and drown out the Biden sham installation
4. Project force to show the hoi polloi that the radical marxists have taken over and cannot be stopped, that our patriotic STATE guard units from every state have been co-opted and assimilated
5. There are probably a few additional more-subtle reasons I haven’t figured out.
NG troops firing on civilians would have resulted in catastrophic fail for the Bidenites.
I think it's to promote the narrative that Republicans are domestic terrorists that require the government to protect society with highly visible and ongoing military force... because the riots may start at any moment.
You know, martial law ... for our safety.
I suspect #3 played a much bigger role than they will ever admit. Even at his drive in events, he couldn’t get more than a few dozen cars at best, and often times Trump supporters outnumbered supporters outside the venue.
Trumps final speeches got more views than Biden installation videos on the alphabet networks. They knew almost no one was gong to show, so needed a reason to keep others out.
“3. Make sure a HUGE Trump rally did not assemble on the mall and drown out the Biden sham installation“
I believe the above was the number one reason for these actions. Had Trump supporters been able to head to DC, the world would’ve seen exactly what happened here.
And just for fun, what did all the conservatives say after the 6th? Dont go, its a trap. Fell for the scam hook line and sinker.