I think the most important factor is that Microsoft puts a ton of effort into backwards compatibility, to make sure that it is always easier/less risk to upgrade Windows (and Office) than to switch to something else.
I know that in the corporate world there are still very many business applications that are mission-critical and very special-purpose (sometimes custom) that were not written in a cross-platform way, and may not be maintained anymore - I would be shocked to find out that it was not this way in the government too.
It is certainly not backwards compatibility. Every internal review of Microsoft systems within the government indicates they upgrade whenever possible, even to the detriment of keeping versions that "just work". That is one of the frustrating things about succcesses in deployment of open source: a mindset that still wants to put out the latest version due to features and "coolness" instead of patching and keeping stability that would ultimately save money.
I know that in the corporate world there are still very many business applications that are mission-critical and very special-purpose (sometimes custom) that were not written in a cross-platform way, and may not be maintained anymore - I would be shocked to find out that it was not this way in the government too.