PHP perhaps isn't quite so well suited to really really big projects. Its object-orientation, particularly in PHP4, is seriously lacking in some respects. It's also a very "ugly" language — thousands of slightly different APIs not very well organised. And I suspect for large corporate projects, it also carries a big of a stigma because of its open-source roots.
Who can say why some things are more fashionable than others though? Java and C# (in ASP) are certainly very capable languages, so it's perhaps not surprising they they're commonly used for the really big projects.
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