Strictly speaking, I don't think Vegas has a low season. It's cheap to visit and stay in because they bilk you on everything else.

The time between Thanksgiving and Christmas is historically slow. Not much going on then, and things are quite cheap usually. Weather is also not miserable.

Getting people to Vegas is heavily subsidized under the assumption that while they are there they will spend rather freely.

The summer is a "dead" season for that specific reason.