RHEL is popular for solutions like running a datacenter mostly because it has a nice enterprise support story. It's what the E in that acronym is for, after all. Ubuntu, meanwhile, is quite popular among us mere mortals who have to fix our own boxen.
Debian is popular for Docker images exactly because many of the people trying Docker were already familiar with Ubuntu. Those users quickly ended up wanting smaller images, making Debian an obvious thing to try out since Ubuntu is basically Debian with bells on.
Ubuntu fought a sea of distros and came out as what's very nearly an industry standard, if not an official one. The 90s were a fricking mess by comparison. Slackware on floppies.
(And now I need "Slackware on floppies" dubbed over the "Jesus wept" scene from Hellraiser.)