I'd say good hw support, no nonsense live installer, and free CDs worldwide got their foot in the door. And 6 months release cycle matching GNOME + 2 months.

Mir/Compiz/Snaps came much-much later (snaps are as much a mistake as flatpak is: they make sense, but are notoriously expensive to make; Unity was a better UX than Gnome Shell 3, but it did not pay...).

However, none of this explains Ubuntu's penetration on cloud servers.

Canonical was actually solving exactly the same problems Red Hat was, just with much lower investment. Their wins made them dominant, their losses still allowed them to pivot to new de facto standards (like systemd too).