Public release of RUMP predated Docker by at least two years, maybe it was more. I just remember when all the Docker hype began I was thinking "WTF, this is not even half as good." But that's what's different about NetBSD. No hype. To answer the question of why it isn't more widely used, IMO, besides absence of hype, it's lack of hardware support, and comfort among those who use it with notions like DIY and minimalism. Often you have to put in some effort to get some hardware feature to work.1 With Linux, chances are someone else has done that work for you.

1 First time I ever tried NetBSD, the audio did not work on the laptop I was using. I added a couple of lines in a kernel header file, recompiled and voila, I had audio. That simplicity made me want to keep using it.