2h left AND right swing are equal or better than 1h left swing:
-All three hit for full damage early in the animation
-2h gets a reach bonus on both sideswings, 1h left swing doesn't
-The "hits the head" feature of 1h left swing is a double-edge sword, because all a 2h user has to do is ready an attack, and their animation causes them to crouch under the incoming left swing. You can't crouch under a 2h sideswing.
So it's not that 1h left swing is so amazing, it's that 1h right swing and thrust are so crap that left swing is often the only option. I prefer to overhead, but that's risky if your opponent is good at dodging.