It comes down to a multitude of reasons actually. (I am an IT teacher at a government school).
1. Teaching IT, we are required to meet requirements for the syllabus. If the syllabus said you must run Windows 98, you had to run Windows 98. Of recent, they've realised the stupidity of it. So they've opened up the options. You may use whatever operating system you want, and whatever software you need, as long as you can complete syllabus. The current language of development in IT is Java. This can run in Windows, Linux and Mac, so not a problem.
2. The Education Department signed an agreement with Microsoft, whereby, as long as we purchased the initial Windows License, we would be able to upgrade to whichever version we wanted for free, as well as give us the full Microsoft Office package for free. Furthermore, we could install 1 server instance for free.
3. The decision to run Windows was initially made on the premises that 80% of the world uses Windows. (See above for reference). However, it does not mean that it is exclusive.
4. The teacher of the subject invariably has only ever used Windows themselves. Having seen the calibre of some of the people attempting to teach IT at the moment, it's no wonder that they are sticking to Windows. A lot of the teachers are still learning how to use the programs themselves. So a new environment is going to be difficult. However, that is not always the case.
The practical exam supplied for IT now includes OpenOffice variants of the MS Office questions. There are quite a few schools out there that have invested in Edubuntu or some other Ubuntu derivative to save costs. Netbeans and JGrasp run perfectly on Ubuntu.
Our school for instance, runs Windows XP across the board as an OS, merely because we have already made the investment, but our next lab will be an Ubuntu setup. 4 of our 10 servers are Ubuntu. The others are Windows Server 2003, merely because the investment has already been made. We have Open Office and many other FOSS packages installed everywhere. Through slow exposure the students are making their own choices as to which they prefer to use. We will educate according to the majority choice. My Grade 11 and 12 classes are sticking to MS Office, but the Grade 10 class want to be taught in both, as they see a future whereby skills in both could be essential.
To clear a few things up. The school gets free Office licenses! The students don't. So we encourage the students to install Open Office, and use it at school as well.
One of the requirements is that each child who does IT as a subject has to have their own PC. By using Ubuntu and Open Office, we are able to broaden the demographic to which the subject is available.
So the issue boils down to what was already in place, and do they have a champion to move away from a monopoly of software choices?