The industry has been pushing HDR and support for variable refresh rate, and that seems way more useful than 4k@120hz, but it's another thing consumers won't be able to notice unless they try really really hard.
Edit: also, does either DP or HDMI support 4k@120hz? I'm still running my 4k monitor at 30hz because my employer doesn't want to buy me a freaking HDMI 2.0 cable!
Of course, 4K 60Hz monitors have been out for years, but are still rather expensive.
There was little to no demand for displays above HD before Apple gave devs the incentives to make their apps support HiDPI by introducing retina displays. Windows took really long time to offer similar support. Linux was (is?) waiting for Wayland to solve this issue.
Look at how much more you can buy in a TV set at a given price than you can buy in something called a "computer monitor"