I recently read an excellent article about the HST in Ciel et Espace (it's in a few days its 15th birthday in space: april 25th).
The article is called "Enquiry on a dream machine"

The article starts this way:
By losing Hubble, the US will lose their best science embassador. Proclaimed "charm" (mascotte) or even planetary hero, Hubble hasn't stopped thrilling the crowds. What's responsible for it's success? A prodigious communication machine, based on images....

Saving Private Hubble...that's what president Bush announced on february 7..

The point is that Hubble is a "star"; its pictures are everywhere, magazines, stamps, CD's...
The HST had a bad start, but was saved in december 93 by what remains the most mediatized Shuttle flight ever. This is the kind of story the public loves.

But (I made a speach about the HST and some guy asked this very question) did the HST produce breakthrough science? Did the HST revolutionize our knowledge of the universe?
I'm afraid it did not.
Why? Because the science teams who were allowed to use the HST for their research had to go through a commission that analyzed the worthiness of their project and that commission is by essence very conservative (the HST is a serious science engine and not just any project is allowed). What the HST essentially has done, was to produce better results if facts that were already known (fine tuning the age of the universe, for instance; better pictures of galaxies further away).
One of the most important astronomy discoveries made during the operational life of the HST was the detection of exoplanets. This was NOT a HST discovery.
Even Perlmutter used the Keck before the HST to confirm the accelerating nature of the universe.

But we all know the HST pictures; they're beautiful...but they're not what the human eye would see. It's largely due to much more PR around the HST than around the other telescopes that we feel the way we do about the Hubble.

Raoul