I don't use web programming technology that is not FOSS running on a FOSS platform, and there are many like me, so for us - ASP .NET just isn't an option. There are enough expenses without needing to worry about licensing fees.
Websites created in PHP are light, attractive and smooth for visitor. Beside it, also suitable for web-hosting.
One of many arguments: Most people use PHP, so therefor there is also most tools, scripts, resources etc...