I think the enterprise IT way of doing things is "pick one and stick with it forever".
Its more a case of Problem Management (ITIL) having not that many calls. And that a respectable company wants at least a CMMI level of 2.
Those two requirements make sure that changes in the IT infrastructure don't happen, very often.
Companies will only select another operating system or application sets if that clearly improves overall processes.
In the case of switching from IE to Firefox, that means infrastructure management have to make changes to application distribution, the whole service-desk department has to be instructed to deliver support, and the users need proper instruction.
A whole new operating system radically different from the former one means a complete overhaul. The changes made to the company to use Linux generally don't outweigh all the work the IT department has to perform to maintain and fix all the bullshit that Windows gives them.
It's like changing from petrol to gas in your car. Once you have gas, it's far more superior to petrol, but getting a gas tank in your car means a significant investment.