Multiple factors: Ubuntu had the Linux for human beings slogan, and at the time that was super important. They promised to create a distro that normal users can use, in contrast to the elitist attitude that prevailed in other communities. That was heavily reflected in the mainstream press Ubuntu reached seemingly effortlessly.

The slogan was also taken seriously. If you asked in my local Ubuntu community (UbuntuUsers.de) there never was a RTFM or a google link. You got help, even if the question was asked already a hundred times that day. We combined that with writing the best linux documentation that exists to this day, but that's specific to the german Ubuntu community.

Also, the installation was easy, it had a good installer. It wasn't even necessary to burn a CD, you could get it shipped. Not everyone had DSL back then nor CD-burners (well, that's true today as well, but now we have USB-boot). That sure helped.