Las Vegas has ownership, an arena, a seasons ticket drive that exceeded the 12K required, and more importantly, $500mm in cash to split with the other owners. Plus, the league can say "we were first" in Las Vegas. And not for anything, but Las Vegas is one of the top 10 TV markets for NBC/NBCSN (something Phoenix, the Raleigh-Durham (where Carolina plays) and Seattle can't say. Which means people there are watching hockey. Las Vegas is becoming a corporate player (especially American defence contractors and the financial services industry) which is where you get your corporate base.
Quebec City: the exchange rate is a problem (salaries are paid is USD, revenues are collected in CAD). Plus, if a Quebec City team goes in, that other team in the province (MTL) is going to want a checque for territorial infringement (and probably a big one). Plus, there's no corporate base to speak of in Quebec (the people who buy club seats, suites, and sponsorships that a club needs to be profitable) other than Quebecor? One horse towns are bad options. Yes- they have an arena and probably an owner but the exchange rate is problematic.
Seattle: No arena, no owner. The company I work for looked at the economics of Seattle for potential deals, and we were unimpressed. I can't go into specifics but employment is shaky at best, and the likelihood of a recession (locally/regionally) is strong (employment has stagnated and is likely to dip (albeit slightly) the CAD exchange rate is hurting them as they have heavy trade with the Vancouver area). Oh, and no arena, no owner and thankfully, a city of taxpayers unwilling to light tax dollars on fire (otherwise known as publicly funding an arena). Not every city is populated with rubes like you get in Pittsburgh (their residents are currently footing the bill for 3 sports facilities with ZERO economic impact) or St. Louis (still paying for a football stadium that left the area), or Miami (will end up paying $1.5bn for a baseball stadium that hasn't created any of the projected economic activity). So who's going to buy a team, buy an arena, convince fans to buy tickets, and make a team economically viable (and pay a territorial infringement fee to Vancouver). An expansion franchise is $500mm. A new arena is upwards of $500mm. That's $1bn. Figure at least another $500mm to get the franchise up and running, get people hired and hopefully be able to absorb losses. That's $1.5bn.
I'll point out that anyone saying a stadium will produce X dollars of economic impact is lying through their teeth. Stadiums have not, do not, and will not produce an economic impact. It simply shifts impact from one venue to another (so people who might buy show/movie tickets instead buy sports tickets). There isn't one study in the last 50 years that can make this claim.