PHP will run on essentially any server, for free. That's a fairly compelling feature for many folks.
There are lots of pros and cons of both, and it certainly doesn't boil down to scripting vs. compiled (incidentally, opcode caches like APC and things like Facebook's HipHop even the score on that point).
I'd say if someone's recommending PHP over ASP.NET, they code primarily in PHP. If they're recommending ASP.NET over PHP, they code primarily in ASP.NET. There's probably not much more to it than that in the responses you're getting.