Using a swingtimer is not difficult. Especially in 2018. Literally your entire rotation is just pressing 1 button every 3 seconds. That seems infinitely easier than having to manage 3 abilities + Heroic Strike.
Again, if 2h really was better, everyone would be using it. But they're not. I've never heard anything about 2h Slam being better, much less about it being "the strongest DPS spec in vanilla".
Let me guess, on 1-minute Molten Core fights where DPS numbers are inflated by crit RNG?