The omission of namepaces in the case of controllers was a calculated decision. PHP's implementation of namespaces is... inadequate, and would have required annoying workarounds, so prefixing controllers was the better evil.
How is PHP's namespace implementation inadequate? Use-as statements are the staple of every PHP library that uses namespaces, and this seems like a half-measure to placate less-experienced programmers.
The omission of namepaces in the case of controllers was a calculated decision. PHP's implementation of namespaces is... inadequate, and would have required annoying workarounds, so prefixing controllers was the better evil.