In addition, with all Microsoft’s faults they had a hell of a small business package for years. In a lot of small businesses, the current CIO came up during those times and dictates policy.
Plus there are a lot of VARs and MSPs who push MS due to favorable terms and kickbacks. Small and medium sized businesses who outsource IT go with whatever they’re told because they don’t have the expertise, time, or desire to explore alternatives.
Plus there’s a load of self hosted software for certain industries that only works on Windows servers.