Henry Spencer wrote:
Mostly because it was already too expensive the first time. There *was*
going to be a similar (not identical, but similar) spacecraft built for
the CRAF comet-rendezvous mission. But CRAF was canceled to save money,
and Cassini itself was cut back in certain areas. Cassini owes its
survival to Huygens, pure and simple -- Cassini too would probably have
died, had there not been a major international commitment involved.

(And alas, Huygens is really quite specialized to Titan. There's no
comparable target at any of the other outer planets.)

Or any planet really. There are only 4 rocky planetary
bodies with atmospheres in the Solar System (Venus,
Earth, Mars, Titan). Huygens might do well as an Earth
lander, or a Mars lander, but in either case it wouldn't
provide much new data. It probably wouldn't survive
long on Venus, so it would be a short-lived atmospheric
probe. Which is what it would be if used on any of the
gas giants. It's a shame really that Cassini didn't
have a Saturn in-situ probe. It could have been very
much more sophisticated than Galileo's, it could even
have provided imagery, most likely. Plus, it would be
very useful to have everything on the table to hit
the other gas giants with. Or, for that matter, to hit
the Jupiter and Saturn multiple times with. The
atmospheres of the Jovian planets are really very much
more dynamic than anything else in the Solar System, a
single probe really tells us very little.

Maybe somebody will design a low-mass balloon borne
jovian atmospheric explorer craft that can hitch a ride
on JIMO or somesuch. Would be nice...