But (just a guess) I hypothesize 4K comes from the film industry, while 720p and 1080[ip] come from TV and computer industries. So the two systems of measuring resolution were created independently and both have historical precedents, and the marketing mishap stems from blithely conflating the two. So I guess my explanation is: coincidence
So I guess we're just seeing a conflation of cinema, TV and computing, at least when it comes to displays and resolutions, so the marketing terminology is conflating too.
Especially since 4k is an exisiting standard, I'm willing to give them a pass for keeping the naming convention. (Although the 4k TV standard, which most 4k monitors will be using, is slightly different: It's the cinema standard cropped to a 16:9 aspect ratio.)
[1] http://en.wikipedia.org/wiki/Digital_cinema
[2] http://en.wikipedia.org/wiki/Digital_Cinema_Initiatives#Imag...