Hmm, the current answers center too much on technical aspects, and the readability pros/cons (which is an important point). So let me try to shift it a bit more onto the PHP environment/community:

• PHP is Perls little stepsister. And an integral part of Perl are regular expressions (they invented that stuff, didn't they?). Therefore it's one cause why regexps are pervasive in PHP too.
• The use case of PHP is coincidentally not much unlike the use case for regular expressions. PHP is structurally used for glueing together HTML pages. And regexps work on text. (what WReach said)
• Micro optimization. As mentioned before: people use regexps and/or PHP string functions frequently after perceived speed. A core problem in PHP circles, not specific to regexps.
• Regular expressions are built-in. In Python, in Java, in C#, in Ruby? there is availability, but a deterrent in having to load an extra module. And see how in PHP or Javascript where it's a core feature, the usage pattern differs. Another exhibit: CSS where it's getting more frequently used.
• The PHP manual is at fault. It often is. Regular expressions are easily discoverable, and I postponed this fun fact because it's boring in its obviousness: all the damn tutorials and PHP introduction books always teach about regular expressions, but fail to educate on use cases.
• The string API in PHP was designed by the same people that brought you magic quotes and the namespace \ separator. It's encompassing, better than Java, but not glamorous in its entirety. Particularily if strings could double as objects (see Python), string functions might outdo regexps.

But that just as side notes. I believe it's anyway mostly perceptional and technical reasons that lead to overuse and/or shunning regular expressions in general. Yet PHP and its userbase has a few properties which compound it, and why we see more questions on SO about it [citation needed!] and they are "morbidly attractive" there.