Philo and Josephus both stood up for the Judean communities in Alexandria at different times. Both perceived themselves as devout Judeans. Both promoted their Law as somehow superior. Bit both were preserved by Christian copycats rather than Jewish ones. 

The best reason I can postulate for this is that their methodologies were considered Hellenistic rather than properly Judean. Philosophy and historiography were worldly wisdom, the wisdom of the Helenistic Age that would be replaced by the Judean Age at the appropriate time.