The size and quality of the market would have to be one of the major considerations any sporting team would evaluate before moving to a city. The size of the market probably does not justify a pro team. The issues of quality of the market already been well stated.
Raivo makes a good point, recently we had an NBA All Star game in Las Vegas and it was as though all hell broke loose there were loads of gun shots, gang activity, etc, etc. and this is not the sort of publicity that Las Vegas needs or wants.
On the other hand Las Vegas does have the one-time/periodic competitions in boxing, MMA (mixed martial arts, etc.). They're big draws...people root for the individual, not the team...and the events are big enough that people will fly or drive in to watch.