Licensing would be the number one reason. Also, I think the devotion to OSX and Apple hardware amongst startups and web programmers is sort of like a survivor

Licensing would be the number one reason. Also, I think the devotion to OSX and Apple hardware amongst startups and web programmers is sort of like a survivor bias. You don't hear anything from web programmers and general software developers that work at the Fortune 500 on what they use. One piece of tech that makes the OS of your PC irrelevant is Vagrant and VM software (and other similar). Build locally but use the same OS as your Production environment.