PHP has the advantage that it is very widely supported by cheap webhosters. You can rent some PHP-capable webspace for peanuts. And that webspace can also host your website and the game itself. The setup of most PHP applications is equally easy. Just copy the files to the webserver and you are done. This makes it very easy and convenient to get a testserver running. 

A C# gameserver, on the other hand, can be more complicated. You need to rent an actual rootserver, which is more expensive and requires far more work and know-how to set up and maintain properly. You didn't write if C# means an ASP.NET application running on a webserver or a C# standalone application running as a separate process. If you mean the latter, you *also* need to install or rent a webserver for hosting your website and the client-sided parts of your game.

That additional overhead can be a barrier for most hobby developers, which can be a good reason to pick PHP. But when a hobbyist is successful with their game and decides to do this professionally, they already have their existing codebase and certainly won't start from scratch. So the will keep their LAMP technology stack.

But remember that if you look at the total project cost of developing an PBBG, then you will usually  notice that the costs for server hosting and administration during the development phase are just a tiny fraction of the development budget. And when you finally go live and have thousands of simultaneous players, then cheap-o virtual webhosting won't do it anymore and you will need to administrate your own LAMP servers which will cause you just as much headache as a C#-based technology stack would. 


So your decision which technology to use should be driven by which technology allows **you** to create the best game in the least amount of time.