Well I can think of a few major things that's keeping Windows dominant.
Many enterprise-level software requires Windows. I mean yeah there are alternatives to Microsoft Office Powerpoint (like LibreOffice Presenter or whatever it's called) but Microsoft Office Powerpoint just makes making powerpoints so convenient, also because most/all company systems are already on Linux it has a higher barrier of switch to Linux (especially when other people are already used to Windows).
Going another level deep, for Civil Engineers (such as myself) AutoCAD, MOUSE, Mike URBAN, HEC-RAS/HEC-HMS, etc. tools all require Windows to run (and Wine doesn't cut it most of the time). There's no good alternative out for Linux which sucks. This is actually probably why I'm still using Windows, because of these software limitations. I mean for the general users and people focused in IT it does everything you need, but specialized individuals don't have the convenience of a product in linux.
Also, videogames. Just putting it here. videogames.
Security may be an issue, but when you have no better alternative you have to tread lightly to make sure it all works and gets you from point A to B no matter how battered up you get (because there's no alternative path).
This sucks.