Here's an honest opinion of why necro has risen so high lately. One word. Resilient. They have in school access to the most infinite armor creatures of any other mage in the game besides the warlocks, and his mage abilities support those creatures better than the warlock. The fact is that damage in this world is usually applied with dice. More dice=more damage potential. So the necromancer has a large suite of creatures that completely ignore 2/3rds of all dice thrown. Luck is immediately on the side of the necro. Add poison immunity and now neither he or his zombies take DoT, at least not the most efficient ones. Piercing, poison, psychic, bleed, sirens call, cripple, pacify, songs in general, all of these have absolutely no effect on zombies. Skellies too except piercing works on them.
These are just facts. Necro creatures are by and large under priced for what they do. Cannot be reliably killed, and can be brought back infinitely (eternal servant). And we have what? Fire for burns, and acid for corrode. Nothing but acid ball does a reliable amount of corrode, and even them out only rolls 2 dice. Necros are popular because they are strong, but moreso i feel because zombies are a tad OP, entirely due to resilient.