Even Microsoft has stated that the majority of the servers being run on Azure are Linux. An MSP usually manages small businesses and maybe a couple midsized businesses. And then ask the doctors clinics, dentist offices, etc.
Most small business owners are stuck in the early 2000s “can’t use some weird open source commie crap made by some kids in his mom’s basement to do real work” mentality.
I worked at an MSP myself for a number of years and the fear of anything not Microsoft was palpable.