The most honest answer is that we've got a ton of Linux servers with well understood and managed configurations, and then these brahmin storage servers that fall outside of that discipline because they have very different update cycles and toolchains. It would be more convenient for us if we could extend our internal update cycle and maintenance to the storage system as well.

There are a number of the usual installation and adoption issues with FreeBSD and Illumos / OpenIndiana. I love the FreeBSD community, and using dtrace again is a joy. I just don't like having to use them for things I already have solved in Linux or can quickly find documented on either Ubuntu or Arch's wikis.

It's nice after the OS wars to be able to say "well, it's easier for us to use linux because it has a broad user base." :)