NewsFactor has the obvious story about why Apache 2.0 is not widely adopted yet:
Another reason Rackspace is not rushing to switch is the lack of module support for Apache 2.0. Many of the standard modules used with Apache 1.3, such as PHP and mod_perl, are still considered “experimental” in the latest version. According to Elmendorf, “Apache is great, but it really serves as a platform to build all this other stuff on…. When you look at a core module like PHP not being supported, there’s no rush getting into it.”
[via NewsForge]