Because I tend to look for/play the most effective max damage templates per class that I can. It has by far the best synergy with Magery. Mysticism by comparison has terrible synergy with Magery and is a highly defensive playstyle, it works best in PvP, and if it was only in terms of PvP that you were asking I would say that these days Mystic/Mage is more effective than Necro/Mage, but only in PvP. In PvM it is a lazy choice for people who want to sit back and let the game play itself (i.e. summon and forget). Where as Necro is heavily involved and works best when the player knows how the game works and reacts accordingly.
Necro has the best synergy with Magery in terms of damage types covering all of them. Mysticism has too many useless chaos damage spells where you will end up targeting the highest resist rather than the lowest.
Necro has the ability to drain mana by spell damage.
Necro has the ability to increase spell damage, and curse effects.
Necro covers more ground with summons/animates. You can have 2 Skeletal Dragons with Command Undead plus 3 animated corpses. If you don't have the charmed undead you can use EV's and cover twice the ground on things like spawns.