I would say because it's easy to use and, at the moment, it's easy to replace developers.
Heh heh. All languages have a particular shelf-life I suppose, but some stay more relevant than others. Many factors will decide whether a language retains adoption rates and continued support. My own experience was with VB, Pascal and a few others before committing to web-based PHP. I've seen deaths a-plenty. I can't see PHP disappearing anytime soon. When it burst onto the scene senior programmer would scoff at it for being a bit lame and not fully functional wrt OOP. Since then, it has matured and responded to feature requests with each major release. It also has the advantage of having a number of big hitters on its side, like Wordpress, Joomla, Drupal, phpBB and a plethora of other apps. It is also pretty platform-agnostic, making it available on Linux and Windows with a range of webservers. It does its server-side job very well - it has a particular niche - it does not try to be all things to all situations (e.g. native app, client-side, desktop). It has huge open source support, including development tools. It doesn't need to be compiled every time you make a change. It has a very accessible syntax (as mentioned by cereal) and you can programme it via OOP or procedural means (or both!). This IMO, gives it the edge over some of the competition. However, one shouldn't put all of one's eggs in one basket, neither should one ignore the convenience and usuability of new, complementary or alternative languages. It's great for now, and that, I suppose is as much as you can say without a crystal ball.
It is not easy to kill a language with such a big community and large code base such as PHP. Because with large code base, there is large demand for PHP programmer and with large demand, there come the large supply.
PHP is not a bad language. It is easy to learn. It is more forgiven to programmer mistake.