Maybe Blizzard has made significant Item Changes. Or that your favourite Necromancer Build is not actually that good anymore.
It could be that your class of choice is no longer the Best Class In Diablo 3. We're quite sure the demonic hordes of Diablo didn't get tougher overnight, so it's more than likely a patch has made some tweaks to the game you know and love. As such, it makes sense that you might be wondering what’s changed within the game. Welcome hero, if you're reading this then there's a very good chance that there's a brand new update in Diablo 3, or potentially a significant patch released onto the games PTR.