There are compounding reasons it takes so long. Some times it just has to get approved by 10 different people before it actually can happen. Many times banks are just the "middle man" and they have to get approval from some investor that could be miles & miles away. Sometimes, they take their time on purpose hoping to get better offers. Many times, they order a whole new appraisal, and the appraiser just happens to be backed up and isn't able to get them an appraisal for a couple of weeks, so essentially they won't even start the process until they receive an offer, then they won't do anything until they receive at least 1 appraisal and sometimes they get multiple ones, THEN they go through their paperwork and try to determine if the loss they'll take on the books is better than taking over the property and trying to sell it themselves.
As one banker told me during a Short Sale Transaction after I sent him all of the sold comps on a house we were trying to work, on..."We'd rather just take a chance that you're wrong by taking it over and trying to sell it ourselves instead of taking a steep loss up front."