I think it has to do with their familiarity with Ubuntu server and the popularity of the distro.

They wrote on that post that they do indeed want to support other distro's in the future. I suspect they just want to get some stability before committing to a wide range of distro's. Ubuntu is very popular, so it makes sense.

Distro's can take the .deb files and extract them through their package managers to fit their systems, probably without much hassle.