The preferences reason is the main one. We regularly run surveys in the community about distro preference, and we haven’t yet seen any one distro get over 30% of the vote. In response to that, we’ve written guides for setting up the top 3 most popular distros and ship the DIY Edition without an OS installed. That’s not to say we won’t ever have a pre-installed Linux option, just that our focus has been on making it easy for people to bring the distro they want.