I think the "Sheldon Cooper defense" is, amusingly enough, the reason.
In the show "The Big Bang Theory", Sheldon, having been issued a ticket for running a red light by a camera, asserts that it is a violation of his 6th amendment right to be "confronted by the witnesses against him".
It may well be that this is one reason why traffic camera evidence alone may not be enough to convict someone. The founding fathers were some seriously sharp guys! Bravo.