Windows 7 allows to run other programs. And users are more than happy to pay for this.
I've actually tried to migrate a couple of mid-range (50-100 users) organizations to Linux. We've tried pilot projects with just a few users. Invariably, it turned out that some small but critical app they are using since 90-s can't work in Wine. Then they try do arithmetic:
> 1) Maintaining Windows + Office licenses costs $100 per year per user.
> 2) Writing a new application costs $10000 - that's 3-4 years of Windows 7 licenses.
Large companies might decide to go on with the migration. Small companies usually just continue with Windows.