Probably because most people post monitor related stuff in the Video Cards and Graphics forum.
Plenty of people have 4K monitors, especially with Dell's new 24" and 27" monitors which were $400 and $500 during Black Friday, and the Acer B326HK which was $699 during Black Friday. Also the Benq BL3201PH is available now at $899. Still pricey if you are comparing to a cheap 27" 1080P IPS monitor for $250, but these are much better pricing overall compared to a year ago.
It makes current and last gen games look incredibly sharp. For example Final Fantasy 13 looks bananas at 4K and it's a old PS3 console game.