[jonnathanson]

There are any number of possibilities, all of them seemingly more likely than the "Boskop Man" theory. Hydrocephalus. Deformities. Even intentional cranial manipulation to produce specific head shapes.[1]

Cf. the "Starchild" skull that conspiracy theorists still love to speculate about, long after it was shown to be the skull of a hydrocephalic individual.

It's not impossible that this was a lineage of large-brained hominids, distinct from our own. Evolution favors local optima; it does not follow a teleological pattern. If big brains weren't advantageous to the local circumstances, they wouldn't have survived. But the big-brained, distinct-species theory speculates quite a bit on very little evidence.

[1]http://en.wikipedia.org/wiki/Artificial_cranial_deformation

[joe_the_user]

And another possibility is that the large brain size just didn't result in greater intelligence or a larger amount of the right kind of flexible intelligence. One could guess that humans need a properly designed brain to behave as we do, not simply any old larger brain.