Lack of real need I think.
Most of the computers in the world are either dedicated embedded controllers or end user devices. Concurrency in embedded controllers is pretty much an ordinary thing and has been since the days of the 6502/Z80/8080. For end user devices the kind of concurrency that matters to the end user is also not extraordinary, plenty of things happen in the background when one is browsing, word processing, listening to music, etc.
So that leaves concurrency inside applications and that just isn't something that affects most of the end users. There really isn't much for a word processor to actually do while the user is thinking about which key to press so it can do those few things that there was not time for during the keypress.
Mostly what is needed is more efficient code. Niklaus Wirth was complaining that code was getting slower more quickly than hardware was getting faster forty years in 1995 and it seems that he is still right.