When I started to work on these projects, there was no other place for me then to develop them as part of PHPUnit. [...] But the more I thought about it, I realized that these features do not belong into PHPUnit but into a suite of tools that PHPUnit is a well-integrated part of.

He did, however, include it as a part of the PHPUnit 3.2 release at that time. Now, however, there are the tools and platforms to make those tests useful outside of the PHPUnit environment and is allowing him to move it out from the testing application and on to closer integration with other software.