The Las Vegas Metro area is 30th largest in the US, with an estimated population of about 2.25 million in 2012:
https://en.wikipedia.org/wiki/List_o..._United_States
Raleigh-Durham, Nashville, and Buffalo are all smaller and have NHL teams (Nashville and Buffalo both have NFL teams too). Raleigh-Durham, Columbus, and St Louis are the only US NHL cities without a NFL franchise in its metro area next season.
Also, remember that it took 20 years for the NFL to return to Los Angeles, and that was with a lot of people trying to get it done, and none of the stigmas that Vegas has. It's a long way from a sure thing that the NFL will go to Vegas.