Well, let's admit it up front that many banks are just overwhelmed with the volume. Banks have never been organizations on the front lines of change and they certainly are not now (I know because I worked for major banks for 23 years). 

Banks have two types of loans:

1) Loans in their own portfolio.  Loans they "own"

2) Loans which they made and continue to service, but which have been sold to investors.

Decisions about type one loans can be made by the bank itself without consultation with others. In this case any hold up is simply the time it takes for the banks internal wheels to grind to a conclusion. 

Decisions about type two loans are far more complex.  Because the bank no longer owns the loan, it must get the agreement of the investor(s) to which it sold the original loan.   These investor(s) may have subsequently sold that mortgage to another investor, in some cases a foreign government. 

Private investors (and any one loan may have several) may have good reasons to want to take the loss in a particular quarter or other time.  Foreign governments may have political reasons to agree, not agree, or postpone a decision altogether. 

Obviously going through all that for type two loans can be very time consuming.  Also, if there is more than one lender (typically a first mortgage by one and a second by another), the second lender, which may have all the same considerations, must also agree. 

It may help sellers, buyers and their agents to learn the ownership of the mortgage(s) before attempting a short sale.