I used Linux because it was more convenient. I was writing a lot of code that had to run on UNIX systems, and it was nice to be able to write and compile it on my home computer. I also had better connectivity; the Windows terminal programs I had at the time were quite lacking. I did use Windows for a while in the summer of 2000, when I had a job writing code for Windows and Macintosh.
Qualifying the reason I switched back is harder. I had an interview with Microsoft in 2001, and although I didn't accept their offer, I was quite impressed by the people I met while interviewing. So after I got frustrated with the distribution I had been trying in 2002, I decided to give Windows a try again. Windows certainly isn't perfect, but overall it has been a much less frustrating experience than Linux was. A big part of that is Cygwin, which has helped smooth out a lot of the rough edges that Windows has. My regular environment now includes the Windows port of Vim, Cygwin/X, and VNC, but I still find that Windows is more convenient than Linux is.
I no longer have Linux installed on either of my home computers, but I still use Linux almost every day at school. The biggest reason is that rebooting annoys me, so since I completed the switch back to Windows, I've rarely used Linux at home. I miss it at times, not so much since the connectivity of Windows to Linux is good, but there are still a few things I can do better with Linux. For example, gcc on Linux is more compatible with gcc on Linux than gcc on Cygwin. I'd really like a low cost virtualization option so that I could run Linux without rebooting.