Sure, you can mandate Linux within an organization. My company even provides migration services for that.
However, in the course of migrations and subsequent support we found out that it requires more hand-holding than Windows. Some required apps don't work on Linux, some programs break after updates, there are still problems with hardware (even very common, like AC97 sound cards). Localization support is still poor (not everyone speaks English). There's still little support for centralized management and so on.
In short, it's simply easier to pay $100 per year per user - that's the price for corporate subscription for Windows + Office + Windows Server.
For home users it's even more clear cut - not much games on Linux, no Photoshop, no decent video editors for beginners, etc.