Because of vendor lock in. Companies that have used windows for a long time have software that is essential and not available on Linux. Replacement is hard, you need to be really committed to get rid of it.
Also, some departments depend on Microsoft office and excel. I used to work in a company that managed to get rid of 95% of all office installations, but after a management change controlling leaked excel back into the company and now it’s back to 100% Microsoft.