Most of the black college QBs who are switched to different positions are smurf sized like Randel El, or were very inaccurate with their throwing. Additionally, contrary to the "cannon arm" every black QB is supposed to possess, many have weak arms. Another reason was there was no hope that they would be able to decipher defensives at the next level.
Those are very rarely the reasons why white quarterbacks have been forced to change positions. Outstanding running ability (Eric Crouch, Scott Frost, Bradley Van Pelt, Steve Bellasari,Matt Jones, Drew Bennett, etc.) is the common denominator.
So the exact same ability -- very good running skills -- that has been used to justify trying to make black RBs into quarterbacks, has for whites been used as a liability to force running white quarterbacks into trying to become wide receivers, tight ends and safeties. It's obviously a very racist practice, but because it occurs against whites, no one in the media would dare speak out about it, and 99% of the fans take their cues from the media and accept whatever explanation is fed to them by the authoritative "experts" in the media.