Because it is modular, designed to be upgraded (the upgrades are ready to go) and it sees in bands nothing elsed does or can on that scale. Will be a terrible loss to science. I just don't understand this hatred for Hubble. We spend more mony on barbeque sauce in this country yearly than a Hubble repair. (Would not surprise me it the same went for Twinkies).
Can't we "sell" or heck, give the Hubble to the Russians? Or perhaps some private group...