I really don't understand why (in terms of number of users) Debian-based wasn't the number 1 priority, let alone why Gentoo was given preference to it.
I'm most likely misunderstanding you, but a debian package (though I think geared towards Ubuntu) was the first thing released, in fact a lot of those listed in this wiki page are built _from_ the debian package. http://repo.steampowered.com/steam/
The linked website is nothing official nor are those builds. I have no idea why this is getting upvoted. Move along.
Valve actually focused on Ubuntu, logical choice.
All of the packages listed there are unofficial. Your question should be "why did a Gentoo user step up and make a package before a Debian user did" The official package is for Ubuntu.
It says "Inofficial ebuilds" under the Gentoo section.