Because of the cast times. Almost everything has a cast time. I knew this would happen. I told people before, that Demo would suck, because HoG isn't instant anymore and a lot of instant abilities removed. Also Demonic Leap is gone too. People told me that it's just Alpha. You think Blizz will fix it until the release? I doubt it.
Sure, Demo will be fun in PvE, but in PvP? We need instants and mobility. Otherwise, it's time to reroll. I'm against playing as FOTM classes, but I don't care anymore. I play as a Warlock, because of the evil theme. Evil people are also strong. Warlocks suck in Legion PvP. Destro is even worse I think. Aff isn't bad, but isn't good either.