Because it's not a real cost savings when everything is figured in...like hiring/training help desk staff to support Linux, or management signing off on risks (or even spending the project time on quantifying those risks).

Enterprise IT is far more complex than most people realize. Sometimes the upfront cost difference is quickly lost in a sea of predictable (or unpredictable) down-stream costs.

Above all, pushing changes with unknown/unvetted risks will be met with a lot of resistance - because risk usually comes with hard to predict costs, in IT terms and in downtime, lost productivity, exposure to legal issues, etc.