1) local prosecution claims that there aren't any grounds to prosecute on ~half of claims that USA wants to charge;
2) defense lawyers claim and popular sentiment (including politicians) believe that USA doesn't have a just/fair court system for computer related crimes;
3) there is a strong position for local jurisdiction in crimes in "cyberspace" - if some action is legal locally, then people shouldn't care if it's illegal in USA; if some crime carries max penalty of 5 years, as is here for computer fraud; then extraditing him for charges of the USA requested 67 years - essentially life sentence - is unacceptable.