According to the Steam Hardware Survey for last month, 47.91% of Steam users are on 1920x1080; adding the 2.56% on 1920x1200 means that a majority of users are still on 2K. The next biggest chunk of users are on 2560x{1440, 1600}, at about 32%. Only 5% of users are on 4K resolutions or greater.

And these are gamers, who are more likely than the average user to care about graphical fidelity. Most of the PC world just does not care about 4K, period.