Starting in the late 90's, PHP (the language) has made staggering advancements toward offering a respectable Object-Oriented model.  The general PHP user community has slowly embraced the changes, but have not necessarily been motivated to conduct cross-paradigm migrations of existing code.  So the procedural frameworks have lived on... and gotten bigger.  But just when it seems that PHP users really have the hang of this MVC thing, SOA may breathe new life into a drooping Dupal or a tongue-tied WordPress. 

To be fair, there is more to survival of CMS platforms like WordPress and Drupal than the PHP user community's stubborn commitment to procedural programming.  Both are useful and widely supported software.  Their shortcomings are in the underlying abstractions.  For example, an application development framework, like Zend, has a more useful tool set for developers to quickly solve problems in a reusable way.  While it is the nature of websites to become more complex over time, developers who have started a project in a procedural framework rarely have an opportunity to revisit that decision.  The result is an accumulation of customizations with comprised abstraction choices on top of the original procedural CMS.  Usually this is accompanied by a bunch of devs saying "If only we knew it would get this complex!" and "Man, this is tough to upgrade!".