In order for a virus to be efficiently spread by a blood-sucking insect,
a couple of things must be true:
(1) The virus must be in high titer in the plasma for a substantial
period of time.
(2) The virus must use the arthropod host as an incubator, rather than
simply as a vehicle.
HBV and HCV certainly do achieve high-titered, prolonged viremia, but
once inside an arthropd vector, they don't survive. This is probably due
to the fact that the virus can't infect the insect's cells.
During most of the infectious cycle of HIV, high-titered viremia is not
seen, and again, the virus is unable to survive in the arthropod.
Concerning Graham Shepherd's response, virtually everyone says that these
viruses are not spread by any insects. If they were, there would be no
risk groups, since we would all be equally at risk of getting bitten.