Because banks are so afraid not just on bitcoin but in all cryptocurrency. If this thing will fully implement in the coming years well the profit of banks will reduce since we can easily transfer money around the world and a bit more faster.
right, banks don't want their money trees to be captured by bitcoin currencies, so they try to prevent bitcoin from developing, but the advances in technology that exist now cannot be dammed by anyone, so the steps that should be taken by the bank are merging and while working with bitcoin owners and perhaps also providing supporting facilities for bitcoin, this might be able to bring mutual benefits.