Portland: PGE Park is so small even soccer's MLS (which doesn't draw the crowds of other major outdoor sports, maybe 15K-20K a game) demanded that Portland build a new, larger venue before the Timbers get promoted. The nearly century old park is built into the ground and has literally no room for expansion. And that discounts the debate over whether there's a market for an expansion team in a relative modest economic region that the Mariners have siphoned regional support from for three decades. You've got to put 25,000 asses in seats every game to succeed: I'm not sure Portland can drum up that kind of interest in what would definitely be a last place team for its first few seasons.
Las Vegas: In case you haven't noticed, the recession and housing crisis has chop-blocked Vegas' economy. On top of that, enthusiasm for AAA baseball and a host of other pro sports ventures has always been weak (Vegas' civic pride is minimal as a city of transplants with allegiances to teams of cities they left behind), and Cashman Field is poorly suited for expansion, as well as barely being a capable AAA venue. And give the summer temps hit 115-120 degrees, a new venue would have to have a roof, making any new proposed stadium a very expensive gamble. And that never minds the sports gambling issues, which may or may not be a real concern. Vegas is a place people go to gamble and be debaucherous... not a place where you can get 25,000 people a night to show up and cheer on a crappy expansion team.
"Virginia": Rarely mentioned whenever "Virginia" is mentioned as a potential market is exactly where in Virginia. Many suggest the Northern area near Washington DC, but eminent domain codes and two MLB teams with stubborn ownership (Orioles, Nationals) make that impossible.
Richmond just lost a AAA team due to a low-grade stadium (The Diamond is seriously just a granite block of stands built next to a baseball field) and barely managed to find a AA team to replace it. Norfolk's baseball venue for the minor-league Tides is AA-quality at best and the Virginia Beach market has never been that hot for sports. There's nowhere to go in Virginia, and nowhere near enough interest to sustain a franchise.
Charlotte and/or Raleigh: North Carolina is actually a hotbed for baseball... college and minor league baseball. The relatively expensive and disconnected MLB brand of baseball isn't assured of playing well here, as pro sports hasn't translated as well to the Carolina economy.
Forays into Charlotte by the NFL and NHL have struggled, and Charlotte lost their first NBA team to New Orleans due to rapidly lost interest (before getting a new team that's had middling results). After a host of newly built sports venues for the aforementioned pro teams, the civic resources are probably too tapped to finance a new baseball stadium, especially after having just paid to build one: The Charlotte Knights just financed a new AAA-quality arena that's nowhere near MLB ready: Their previous facility was actually located far down the road in South Carolina. Not quite up to snuff for an MLB team.
Montreal: The only period where Montreal gave viable attendance support to the Expos was during the early 80's, probably their most successful stretch as an MLB team. After that era sunk, the Expos sunk to the NL's bottom third in attendance and never left, not even during their fool's-gold contention run during the strike-shortened 1994 season. Montreal did get a good pocket of support for the Expos but it was not nearly enough of a fanbase to sustain a team, even before their losing and attendance took a turn for the worst during the late 90's. There's not enough of a baseball market in Montreal.