I can't speak to tournament appearances, but I can tell you why I love the Necromancer. For me, style is a big part of it. I've always thought black had some of the coolest cards in M:tg, and in dnd, my favorite enemies were liches, vampires, and other undead. (And some demons, too.)
So when I tried mage wars, I immediately gravitated towards the dark mages. Warlocks immediately caught my eye with their demons, and a few undead here and there. And then the Druid vs. Necromacer expansion came out, and I still think it's the coolest expansion in the game. There's so much style! You've got shambling zombie hordes, you've got an altar that powers up by placing skulls on it, you've got the book of the dead as an equipment! (And if you have the sweet cardsleeves with the Libro Mortuos art, you can have your spellbook card in the spellbook sleeve, inside your spellbook. It's like a Russian doll of evil!)
Mechanically, the Necromancer does awesome things, too, that tie into the flavor. The poison immunity to me implies that the Necromancer is on the way to undeath himself, and the ability to use it as a weapon with impunity is pretty useful. Having a graveyard and an evil book as spawnpoints are awesome flavor, and it also allows a cool strategic option of using both to just flood the board with creatures. And having skeleton objects that can't be "healed," but can be "reconstructed" with an ample supply of bones is also pretty cool, and useful. I actually prefer the skeletons to the zombies, both because I think they're cool, and because I'm a contrarian like that. :) They actually also work a little better against other necromancers, imo, because you're not paying for bloodthirsty effects that won't matter. And with the dark conjurations that sap the other player's life, there's also an option to effectively set a "clock" and dare the opponent to try to kill you before that.
In short, Necromancers are both extremely cool from a flavor perspective, and have some unique strategic options open to them that separate them from the other mages. That's why I like playing them, and I expect that's true for other people as well.
So when I tried mage wars, I immediately gravitated towards the dark mages. Warlocks immediately caught my eye with their demons, and a few undead here and there. And then the Druid vs. Necromacer expansion came out, and I still think it's the coolest expansion in the game. There's so much style! You've got shambling zombie hordes, you've got an altar that powers up by placing skulls on it, you've got the book of the dead as an equipment! (And if you have the sweet cardsleeves with the Libro Mortuos art, you can have your spellbook card in the spellbook sleeve, inside your spellbook. It's like a Russian doll of evil!)
Mechanically, the Necromancer does awesome things, too, that tie into the flavor. The poison immunity to me implies that the Necromancer is on the way to undeath himself, and the ability to use it as a weapon with impunity is pretty useful. Having a graveyard and an evil book as spawnpoints are awesome flavor, and it also allows a cool strategic option of using both to just flood the board with creatures. And having skeleton objects that can't be "healed," but can be "reconstructed" with an ample supply of bones is also pretty cool, and useful. I actually prefer the skeletons to the zombies, both because I think they're cool, and because I'm a contrarian like that. :) They actually also work a little better against other necromancers, imo, because you're not paying for bloodthirsty effects that won't matter. And with the dark conjurations that sap the other player's life, there's also an option to effectively set a "clock" and dare the opponent to try to kill you before that.
In short, Necromancers are both extremely cool from a flavor perspective, and have some unique strategic options open to them that separate them from the other mages. That's why I like playing them, and I expect that's true for other people as well.