In a lot of cases, there's no "dual license" option - we have to have a set number of licenses for one platform or another. Which would mean adding licenses, since we wouldn't have exact seat counts. We'd have to pad both numbers. And that's for the software that has a Mac version, not all of them do.
With budgets being cut, and software companies raising licensing costs, we're going to be lucky if we can maintain 100% of the licenses we're currently using, never mind adding cost.
So anything that adds cost without adding some sort of direct, broad benefit is really unlikely to happen.