The reason is takes so long is that in most cases the manager working your file is overworked (too many files). The other reason is that the mortgage was sold to an investor, and that investor must approve the short sale, so there are quite a hurdles that must be overcome.
sorry the delay. be patient.
1. During that time they expect you to get nervous and drop a higher offer (happens all the time to first timers)
2. to verify that the owner doesn't have any assets to take when they agree to take less than owed