Great hackers demand control. Great hackers share stuff.
All the cool toys come from great hackers, and they end up sharing and improving on each other.
Think of all the cool shiny stuff that's been coming out recently: Python, Ruby, Rails, Django, Flask, Node.js, etc.
All of them come from people who use a Unix system (Linux or Mac).
Unix systems offer a great platform and great tools for serious programmers to do their work.
Windows systems and Visual Studio tend to be used more by people who just want to "get things done" in the established way and earn some living.
It's not just because they start small as CodexArcanum's answer suggests. It's not like "oh they would love to move to windows but it's too late". They probably don't want to move to windows. It's severely lacking in terms of tools and power for the kinds of things involved in running a web startup.
For starters, most new shiny OSS projects use git for version control, and probably have their official code repository at github or something like that.
To quote Paul Graham:
What do hackers want? Like all craftsmen, hackers like good tools. In fact, that's an understatement. Good hackers find it unbearable to use bad tools. They'll simply refuse to work on projects with the wrong infrastructure.
A couple years ago a venture capitalist friend told me about a new startup he was involved with. It sounded promising. But the next time I talked to him, he said they'd decided to build their software on Windows NT, and had just hired a very experienced NT developer to be their chief technical officer. When I heard this, I thought, these guys are doomed. One, the CTO couldn't be a first rate hacker, because to become an eminent NT developer he would have had to use NT voluntarily, multiple times, and I couldn't imagine a great hacker doing that; and two, even if he was good, he'd have a hard time hiring anyone good to work for him if the project had to be built on NT.