Early on, PHP had the advantage of mod_php, which made deploying PHP applications simple and performed well. Because other languages required more complex setup (e.g. fastcgi, servlet containers) shared hosting providers gravitated towards supporting PHP applications. This caused open source projects that used PHP to gain more traction than projects written in, say, Perl. Eventually, hosting providers realized mod_php was not such a good idea. See the following link for an expansion on that http://www.majordojo.com/2007/11/is-mod-php-falling-out-of-f.... I think the shift of shared hosting providers to FastCGI (which makes supporting additional languages less work) as well as the decreased cost of private servers due to virtualization is playing a role in PHP's apparent decline in popularity.
