As Dries says in his blog:
It seems that we have arrived at a point in which there is a symbiotic relationship between PHP and the most popular PHP applications. A relationship that did not exist when PHP was created. Symbiotic relationships are obligatory: we depend entirely on each other for survival. And yet, I feel like we've been living apart. It makes sense for us (i.e, application developers) to contribute to the development of PHP, and for the PHP core developers to work more closely with the developers of the most popular PHP applications.
He points out that, in Acquia's case, there's just not enough time for the group to contribute back to the PHP project. Other posts surrounding this same discussion have also noted that there's not a good mechanism for people to "give back" to the PHP project and that getting involved requires a certain skillset that lots of developers don't have.