I don't remember why exactly but it was definitely just 'linux VPS = Ubuntu' for a long time. I think they got reputation as being the 'just works' option because they were happier to include some non-free graphics drivers etc where Debian was the more pure option but most end users didn't care
But since they added snap and I started running into issues with trying to install Docker etc a few years ago I default to Debian now and most of the time I don't even know which it is.
And don't get me started with Debian testing. I want a LTS distro not a testing one.
They also had credible commitments to long term support that brought people on board. I think that was enough to get them on the map, but personally as a desktop user, I haven't used it in a while, mostly sticking to Debian and Arch.
Ubuntu found me (finally, with the tutorialhelp of a localLLM) in 2026, and I don't trust most of the opt-outs.