Because the hardware is attractive, it's well supported and the OS is decent, and it still runs all the open source stuff they like, as well as proprietary stuff they like.

I personally run SUSE on a Thinkpad, I prefer Linux to OSX and Windows, and love Thinkpad hardware (the keyboard is the best I've ever used). I can however understand why Apple stuff is attractive.

I've also heard that Linux is easier than OSX when it comes to installing languages, tools, etc..., but OSX is easier when it comes to things like Skype, Office, and of course proprietary software. I've found using Linux full time to be painless and enjoyable though.