There could be any number of reasons. This could even have helped hurt black money, had it been done in a proper manner. Earlier in this thread I have mentioned that a lot of fake currency could have been deposited. So, this exercise could have an effect opposite to the intent - that it could end up as a huge money-laundering exercise, with black money reasserting itself with the help of Rs. 2000 notes.
The Govt. releases some figures related to the deposits and withdrawals. I would also like them to reveal how much fake currency was caught and rejected. It is possible that a large amount of fake currency got deposited because the bankers can't be thorough in their checking at this time.
Also, they should mention if the withdrawals are higher/lower than the normal.