It has nothing to do with monitor manufacturers not being able to do something. Plenty of displays from the Dell 5K to the Dell 8K have multiple combined Displayport inputs. 4K at 120 Hz has technically been possible for many years. For whatever reason, be it market studies etc, display manufacturers have chosen not to do it. cithix has tapped into a real market that display manufacturers either thought didn't exist, or thought was too small to pursue. Display manufacturers get a lot of things wrong and rarely take risks.
Yeah, but I think we'll be seeing a lot of 4k/120 TV's (with freesync) now that HDMI 2.1 is a thing. It won't be for a couple more years probably, but they will be here at some point. But what I want to know is, will they have input lag measured in microseconds? That's where cirthix is really trailblazing