This would most certainly be the "best case scenario."
Sparing you the gory details, with a short sale you are dealing with the present owner who wants their lender to accept less for the property than is owed on it. Banks are not very quick to accept a loss and move on. A more realistic time frame would be 3-6 months...and then the lender can always reject the deal.
Many buyers knowingly elect to avoid these options and seek others.....
The reason for the delay is generally two-fold. Most lenders won't take the time to work with the property owner. If and when the property owner contacts them to explore doing a short sale all they get told is, "Bring us a written offer to purchase and we'll discuss it at that time." Then when they receive an offer they start to qualify the Seller which basically means a full audit of all their finances. Roughly half of all sellers are not approved for a short sale as they have assets they could use to pay the bank off and if the bank discovers these then the seller is told, "No, we're not going to approve you for a short sale." This alone can take 4-8 weeks. Assuming the lender finds the seller does meet their standards for a short sale, then they need to research the property itself, to determine if there are any other lien holders on the property. These could include a second mortgage, a home equity line of credit, lens by contractors, tax liens by the IRS, the state or local municipality. Any other lien holders must also approve the terms of the short sale and any one of them can say no at which point the deal won't go through. This can take another 4-8 weeks. Only then will the bank respond to your offer and there's no guarantee that that they will accept it, they may counter the original offer and now you and the bank have to negotiate in the normal manner.
Buyers considering a short sale need to understand that fully half of all properties listed as short sales never get sold, the bank simply forecloses on them. If you're not prepared to take several months with the understanding that even then the deal may not go through, then you should not be looking at short sales.
2 months would be a dream to most short sales. I've been doing them since 2002 and rarely see them done in 3-4 months. My longest took 19 months, but I've closed all the homes I've ever listed as short sales.
The answer to why so long is up to the banks. Their systems often defy logic and common sense and have frustrated many buyers, sellers and their agents. I did a blog post called The Anatomy of a Short Sale, which may provide some insight into all the moving parts to get to an approval. Best of luck with yours.