Because PHP is extremely easy to program in comparison. It also has, far and away (IMHO) the
best resources and open source material on the internet - someone has
already done most of the hard work for you if you are any good at using
Google
Hosting is generally cheaper too- even premium/dedicated stuff.
PHP is also better understood by the current generation of developers.
I'm a JSP developer predominantly but have really come to appreciate
PHP - things are so easy in it that for small scale apps it is a good
choice. I also think that people overplay the performance issues - if
you have a good dedicated server for PHP it is not going to be much
different in speed than a JSP or .NET server.
If you want to start looking at larger scale apps then I would suggest
JSP/J2EE and .NET offer a better framework for developing, but not
because of performance - having precomiled classes & webpages as well
as strong typing mean that run time errors are much less likely to
happen.