Int, their key stat, gives 5 mana a point, so they have no mana issues to begin with. Pretty much all the mages passives are about getting more mana, raising their ratios on spells, or giving them accuracy. These are not the passives and it is probably why I don't see too many mages. Instead of getting better stats from their passives, mages can get mana which supplements an issue they don't have.
They have enough passives to get an extra 25 mana per int and an extra 20% mana. Assuming 350 int, that int gives you 12,600 mana, why the heck do mages need this? They can also get 35% mana consumption and improved mana regen, but they need none of it. Ok, maybe some of it, but this is a bit much.