My opinion is that he had no GOOD reason. He just knew the Windows dev stack and has a giant ego. But, I'm probably biased since he reminds me of managers/PMs I've worked with in my career that have weak opinions strongly held which affect me.
I think it's similar to how he has banned use of Kanban or other Toyota Production System principles at Tesla (which is the easy explanation for their poor build quality, see: U.S. car manufacturing 1970's).
