There are a couple of reasons why, in my opinion.
The role of ICT Co-ordinator is not taking seriously in a lot of schools. It is literally dumped on a member of staff, usually the newest. There are of course exceptions to this, and those schools are usually the ones that have a good setup.
Schools that buy outside support often change providers every year. How are they meant to get a grip on setting up and maintaining a good network and support provision, if they have a different provider each year? Different people, do things in different ways.
We took over the support of a school in April '09. The network consisted of an awful lot of old computers, especially laptops. There was no funding to replace these. A few weeks later we found out that the school had ordered new workstations for the classrooms. They didn't consult with us, and bought some that we would not have recommended.. Throughout the year, these caused all sorts of issues. Come April '10, the school did not buy our support back and signed up to another provider.
3 different support companies in 3 years. All have different ways of doing things. 3 different ICT Co-coordinators in that time too.