Because with the networks commericals pay for it that's why PBS needs donations
when any provider provides the channel they charge for it.The best HD signal you can get now is with a Over the Air Antenna.Cable and Satellite providers compress
the signal lowering the quality of the signal due to lack of bandwith.
All channels don't have to be HD but they will have to be digital.I believe the date now is by 2009 if you are close enough to a transmitter that will be your best bet
for a high quality High Definition signal.