It's not the building it's the legislators.

Only one reason why would attack a legislature like this while it's in session.

If you look at this as the far right president losing his grip on power and sending his supporters to storm the legislature in hopes of a hostage situation that he controls, then it starts to sounds more like an attempted coup. Massive security failures, obviously sabotaged responses, and finally that same president apparently getting overridden in order to facilitate a better response.

What's difference between this working and not, some additional military placements and a phone call to Erik Prince to get some pros in the crowd?