PHP has the advantage that it is very widely supported by cheap webhosters. You can rent some shared-hosting LAMP (Linux, Apache, MySQL, PHP) webspace for peanuts. And that webspace can also host your website and the game client. 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 (a pretty straight-forward solution for a PBBG if you ask me) or a C# standalone application running as a separate process (a bit unorthodox, but might work quite well if you have a more action-oriented game). 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 they will keep their LAMP technology stack even though their initial concerns for picking it aren't valid anymore (when you are running a serious operation on a LAMP stack you will sooner or later hit a point where shared hosting does not cut it anymore and you need your own servers).
But remember that if you look at the total project cost of developing a 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.
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. Picking the right technology stackbest tools money can buy (best for each projectyou and your project) does not cost you money, it saves you money.