Police are very cautious on their approaches now. The officers at the Boston bombing shot up a boat and waited an hour before approaching it. When Charles Whitmore shot from the University of Texas tower, I believe it was only two officers who stormed the observation deck. Different attitudes, different methods.
There's certainly a time for caution, but if a shooter is continuing to take people down, that isn't such a time. But that's the question: did the concert clear out in ten minutes and he stopped shooting completely, enabling caution? Data needed.
but a number of civilians had long guns in their vehicles and started suppressive fire rather quickly. It was much easier to ID the shooter's roost in the tower than in the hotel.