Flash Sync Speed: HSS is supported on many cameras/flashes combinations
HSS eats power like crazy. You loose three quarters just for turning it on, and then the output stays constant in relation to the ambient light.
The leaf shutter otoh sets the target aperture, giving you a 4-5 stop advantage. Contrasting to HSS you also benefit from ISOs above base sensitivity. You couldn't replicate that with 600RTs because the control system can't handle as many units as that would require. Not to mention the logistic challenges, or the implications when not relying only on speedlights.
You'd need at least 100kg worth of speedlights and batteries to match a single, easily backpack transportable, Move1200 - just for perspective. And that doesn't include stands or clamps. For B1s it works out similarily.