The reason MvC2 is so popular after all these years is because its super enjoyable to watch. MvC2 has had like the top attendence in Evo for god knows how many years.
And I'd say theres like uhh 8 characters or so to pick from: Storm, Magneto, Cable, Sentinel, CapCom, Dr. Doom, Strider (only w/ Doom), Blackheart. and I'm missing a few of the more ghey characters (aka Spiral)
If you're talking about point characters (ones who don't need CapCom or Doom to ****), then theres Storm, Sent, Mag, Cable, Doom, and maybe Blackheart.
MvC2 is very much like Melee in it's appeal. It might not have super high character variation when it comes to tournaments, but it's really enjoyable to watch and surprisingly technical.
Oh hi I'm here to tell you this isn't true. Though it is close.
I don't remember the attendance from all the Evos but I know for a fact that Marvel was a good deal behind both Third Strike and Melee at last year's Evo. And by "good deal" I mean like about 100 people less. I'm also pretty sure that it didn't have the higest attendance two years ago as well, though I could be wrong there.
Most of the characters you mention can be played but have been proven in recent years as non-winners. Heck in the last few years even Cable has been pretty much phased out, leaving Magneto, Storm, Sentinel, and assist characters. Blackheart has been buried for years, only one person can use Spiral, and only one person can use Strider/Doom (and this person never wins).
Plus, no love for Iron Man?
Really at this point you must use two of the following if you want to win: Storm, Sentinel, Magneto. Plus an assist character of choice.
Anyway, I think Marvel matches are fun to watch because of the crowd. Otherwise, most people don't really know what's going on other than "a bunch of **** happened." Though I do agree that Melee does have similarities to Marvel in terms of why people like it. I'd argue that Brawl has a similar appeal to Third Strike, and I know that at least Lucien agrees. I should get back into 3rd Strike.