Because it's easier to deal dmg by pressing 123 and climb to 2k by doing nothing but maintaining dots as aff, it requires no forethought or planning, so the only peeps playing aff in arena are either diehard oldschool fans of aff playstyle who refuse to give it up and new warlock players who rerolled during prepatch or within that time period.
Demo requires a tad more ,let's call it activity , and preparation to throw out a well timed TC.
Don't get me wrong though, I don't feel like any of the current lock specs are complicated, some simply require a bit more forethought.
I can only imagine you don't do arena. Maybe you're talking about 1v1? At which point get out, because that couldn't possibly be more irrelevant. If you're talking about actual competitive game modes though then you're just a liar. Seeing as how there are multiple Warlocks in the top 100 on US I imagine you're just making things up though.