Zar, Chibi, et al:
I think the reason Arcana's enjoyed success as quickly as it has is simply because it's promoted by an SRK mod/Capcom OG. In particular, it's hard to think of any reason Marvel players would try the game, except Art's street cred.
And the mod support creates an environment heavily slanted toward the game in question - for instance, I recall polarity being banned from SRK very recently for some shit talk toward Arcana (standard stuff, too, nothing really crazy).
Heck, Guilty Gear initially got big for almost the exact same reasons, actually, with tragic in Arturo's role. He made his own separate forum for the game, but the rest is pretty much identical.
Melty, on the other hand, never received any support from major Capcom heads (IIRC) until Arlieth came along, and he's not really "OG" like the others mentioned. But if you'll notice, the game has been growing since then...biggest side tourney at Evo, outperformed the pre-reg numbers at NEC, etc. As someone who's been trying to help SNK get the love it deserves, I think you guys should be proud of what you've already accomplished!
And one last note: Just on a hunch, I don't think Melty or Arcana will ever get into Evo. Art would know about this better than I...but I know inkblot has called Arcana "the child molester's fighting game" before, and overall I get the impression that the Cannons feel the same way I do: That the loli games are bad for the image of fighters as a whole, and will ruin their chances of drawing a new audience. Again, if I'm way off on this, feel free to correct me by all means - that's just how it seems to me at this point.
Later everyone,
Josh.