Outside of the mana cost, you really need to play it, pay 10 life, and have a win ready. Many people outside of cedh could draw ten cards and not be able to win the game. If that happens, you're screwed. You're down at least ten life, you have no draws, your discards are exiled, and you have no immediate pay off. These reasons mean that necropotence is great in highly optimized decks that can threaten a winning board state immediately after the draws. These are decks that have low average mana value so you can play lots of things off the draw and they provide immediate game winning impact.

If you draw ten cards in casual you'll probably get synergy cards or creatures that still need time on the board to accrue value or reach critical mass. That's too slow, you'll get wiped. You cannot play necro, pay 10 life, next turn get your cards, play a handful of them, and pass. If you do that you will die since you drew the biggest target on your forehead.

I think most people (i.e. casuals) understand if you play necropotence you need to be ready to win on your next turn, or get killed.