Not everyone is tech savvy enough to use Linux; probably even WIndows. But since Windows is the default system most people used in their past, people would be familiar with it.
In one of the companies I worked, we switched from Windows to Ubuntu for all the support staff (30+ people) because of the cost. However, the productivity of employees fell sharply even with training. We then reverted back to Virtualized Windows set up because the employees were not finding it easy to work with.
In another company I worked (MNC), they stuck with Windows because of support from Microsoft. For them, support from the vendor is much more important than the small cost savings they could have had from from switching to free software.
I agree with the Office Suite comment that @realmccoy made. Libre office is just terrible to use compared to Microsoft office suite. If you are a company that values employee productivity, there's very less chance you'd be willing to sacrifice productivity for some small cost savings (10K per year per employee is nothing for most businesses).
For my level of work, the OS does not matter anymore. It's been ages since I have used a desktop application other than my browser. Google docs offer serious power for most office tasks. I use a Macbook Air and I am not willing to give it up because of the weight and batter aspect of it. If there was a Linux equivalent of it, I would not really mind if company asked me to use it.