The number one reason would be debugging. The debugging environment as it relates to PHP with Netbeans is simpler to configure and easier to use than with Eclipse. There are other reasons but I feel they are more subjective than the debugging issue. I've tried Eclipse many times over the years and have always either gone back to the old Zend IDE (before they moved to the Eclipse platform) or Netbeans (lately) in frustration.

To be honest, I frequently use Vim for quick editing but when debugging PHP Netbeans beats Eclipse hands down.