It's mainly a political thing, for instance the city of Munich started a partial switch back from Linux to Windows and when political parties start to enter, you can bet on it that lobbying by Microsoft is involved. A minority of the users, the cityworkers, wanted the switch back to Windows and Microsoft Office. One of the problems the city had was a decentralized IT department, which can cause problems with updating, but still is not a reason to switch back and could be solved with a few cron-scripts (I say that as an avid and fanatic Linux-user, I am far from being a technician).
Another "detail" about mainly using Microsoft products is convenience, read: being scared for new things or just too fucking lazy to switch to something better, safer. I call that the "Facebook effect".
All in all you can say it is the cause of politics and mainly conservative minds that don't know better or don't want to know better. Like with many problems in the world right now, I dare to say.