Years longer of being a web-focused language than other options.
Lots and lots of really good tools, all easily installable via PEAR or PECL, or on the development side.
An excellent community.
Lots of smart developers.
Lots of support from big companies (Not to mention acceptance at the enterprise level).
Continued development and progress.
Other technologies don't offer any big advantages to switching.
Despite what you might think by reading HN, PHP is still a major player, and still has a lot of momentum going forward. Using PHP for your web site is not the exception.
Meaning; you must go out of your way to find a host which supports Django (not far out of your way, but still)
You would have to go out of your way to find a host which does NOT support PHP.
Or alternatively; WordPress uses PHP, ergo people still use and write PHP.
Isn't that what trolling is?
I'm not sure why that would even provoke rants by language zealots. It's a simple question with a simple answer: big libraries and projects that can't be ported in an afternoon.
That is to say, Haskell is a nice language but there's no Drupal port to Haskell.