1/6 was a much more sinister day in US history than 9/11 can ever be.
If the plotters of 1/6 had been successful we would all be living under fascism right now. There would be pogroms and human rights abuses to this day and beyond against various sections of the US population that would dwarf the death toll of 9/11.
Unlike Al Qaeda, the enemy is within. We don't have the option of fighting them over there. There are millions of terrorists within our borders right now and you know damn well they are plotting their next move.
We have to come together against this internal enemy. Formally commemorating the tragic events of 1/6 every year is just one way to do so.