My own opinion, for what it’s worth, is that the Capitol police were left deliberately undermanned for the situation (I’m sure they had the usual number of officers on duty, just no extra). They then took a tactical decision given the overwhelming numbers not to use force in preventing the protestors from entering once it became clear that was there intention.
Any use of force in those circumstances could have triggered an absolute bloodbath - I have no doubt that some of the protestors will have been armed - DC law does not allow open carry, but concealed carry is allowed if you have a permit. Though let’s face it I doubt most of those protestors are from the DC area, and certainly won’t have been in possession of the right paperwork.