The medium-sized enterprises (100MM$ to 500MM$ annual revenue) are the ones who aren't looking at Linux. These are the ones with in-house tech support, who only know Windows. Maybe they are the average customers that won't switch over?
Note that I'm only talking about cloud-hosted SQL Server installations. When you're buying the whole infrastructure stack, it's a different story. Also, this isn't the area I specialize in, so this is just a narrow observation, not a broad pattern.