HST and astronauts
Astronauts are the best way to service the HST.
This of course means using the Shuttle, but it would have been tested several times before anyway.
It's a matter of various factors: the HST is the Greatest Orbital Device (GOD) of the Shuttle era, providing us with wondrous images. A robot would take too long to develop, be very expensive and there is a matter of time. The HST was meant to be serviced by astronauts.
And it's very psychological: a Shuttle flight is risky but how about the rewards of a successful flight?
Raoul