"Everybody" is not using Linux for their mid-level servers; Solaris, especially OpenSolaris, is still a very viable option, and offers a number of features (most notably ZFS and Zones) that Linux isn't really close to, yet. Not to mention, Solaris has, in my experience, a much better I/O scheduler.
Linux is a fine operating system, but it isn't the end-all-be-all.