I'm hesitant to speculate too much before there's been an investigation, but I believe the shooting took place when there were still a number of congresspeople in the gallery of the house chamber, and she appeared to have a large backpack on that obviously didn't go through security screening.
I can imagine the Capitol Police being worried that something very bad could happen if she was allowed to breach the barrier and run into the chamber.
I can imagine the Capitol Police being worried that something very bad could happen if she was allowed to breach the barrier and run into the chamber.