They existed for a while, and as recently as a year or two ago there was a cheap LG 24" 4k that was only about $300. But I think the monitor market in general moved on to focus more on larger sizes, and "4k" became the new "HD" buzzword that meant most products weren't even going to try to go beyond that. So basically only Apple cared enough to go all the way to 5k for their 27" displays, and once everyone else was doing 4k 27" displays a 4k 24" display looked to the uninformed consumer like a strictly worse display.