Mostly the lore. The "evil" demonic thing sounds kinda cool to me.
Also, I've played the same warlock main since 2006, by this point I can't really imagine changing. I have alts, but they'll never, ever be my main. If the Warlock class was screwed over and made unplayable I'd sooner quit than reroll. I've stuck with the class through the highs and lows, I'm not going to stop now.