Ubuntu - got its fame due to excellent marketing, not so much due to technical superiority. That's all
Debian(-based) - got its fame due to superior package management in the past where RPM sucked a lot compared to it, more precisely the front-ends did. Also, Debian is very fanatic about pure open source and application stability, hence their "stable" branch throws you back in the stone age
openSUSE - technically very advanced and stable (same as Debian), superior RPM front-end manager (zypper) compared to other RPM-based distros, had/has bad
PR due to mother company being faggy and striking deals/patent covenants with the devil which gave bad taste in a lot of people's mouth, especially the fanatics' mouths. Still suffers from disorganization and sometimes lack of vision (ie, is virtually never the first to incorporate ground-breaking new stuff. Often Fedora does it first, then openSUSE follows)
Also, still a lot of people believe the myth that even today, RPM is inferior to Deb which is not really true but they believe it due to historical reasons and current misinformation that's still kept alive