If there is one app that somebody needs, and that app does not run on Linux, then Linux is a non-starter. And many such apps do not run on Linux.
Governments, and corporations, are comfortable using something standard, and corporate supported. For them, the expense of Windows is not a big deal. Governments, and corporations are not concerned with the idealistic aspects of foss. What is a big deal, for them, is the labor pool that understands Windows applications. They wants standards, they don't care about nickel and dime saving, or any kind of ideals. They don't want to defeat "the man" they are "the man."
Government and corporations are a huge part of the desktop/laptop market, and as such, set the standard.