There are few jobs in the banking sector as compared to the government sector. With the bank tax there could be created many good government jobs and public works. The banks do nothing of value except trade money around. They have too much money and can make it too easily as it is.
I admittedly know little about the bank tax, but it would seem that an added tax would only encourage banks to be less concerned with job creation. With less money at its immediate disposal, banks may see the extra tax as a further infringement on their ability to pay new employees.