People are packed together more densely when they are massing to get out of a doorway than when they are separated by seating.
OTOH, maybe the perps were too cheap to waste their ticket money and wanted to see the whole show first.
It is thought the bomb was detonated in an area before a secure area. This would explain why it was done after the concert. Masses of people.
Per FOX’s latest reporting: A report from a US IC contact, reporting to press vector- that there was a suicide bomber.
Fox is now reporting it was actually detonated outside and not in the lobby after all.
Better answer: the bomber didn’t have to smuggle a bomb into the arena to cause mass casualties. He saved himself the trouble of bypassing whatever security there might have been.