From the pictures, it appears that there were only a few hundred people at most.
Many thousands... Too many for the Capital Police to manage. The last thing they wanted to do was attempt to use force against the veterans.
1) They don't have the manpower to deal with them, and many of the vets were prepared for a fight. One group led by several retired Army Rangers suggested to the Police that they would be well advised to step aside. They stepped aside.
2) The political fall-out would be beyond measure, beyond anything most people can imagine.
3) Police at the White House were clearly intimidated, even frightened. Watch the videos. Those cops wanted to be anywhere but where they were.