They wanted those people to be able to get in...and Trump knew it was happening as he watched in his bunker listening to music as if they were at a ball.
It was a planned protest, the whole thing was blown out of proportion and Congress took advantage of the situation and made it all trumps fault when he did nothing wrong
The political agnosticism of the military is important to maintain. We learned during the GF protests that the Guard's mere presence was often inflammatory. Its the only logic I can see behind this course of action.