It's the most popular because it's Ubuntu's default which happens to still be the "default linux distro". Many GNOME users probably don't even know what GNOME or a DE in general is (and that's ok).
If you ask linux nerds, however, KDE generally seems more popular but that's probably because people who care about this sort of thing tend to value customization and KDE's philosophy more than GNOME's.
Source: I use KDE on Arch BTW, guess which category I fall on.
TBF it's likely the most popular because it's the default not necessarily because it is better than KDE (or worse, they're both good just aimed at different people)