Ghosts on VHF won't vary as much than on UHF, especially doe to broadcast towers bouncing around from wind. That tower shake can make the broadcast antennas move a substantial fraction of the carrier wavelength on UHF vs VHF. That means that ghosts at receivers are varying too quick for most DTV receivers to process out the ghost conditions.
Of course the main motivation was mentioned in a post above, to get paper coverage and cable must carry at low electric bill costs.
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