The diagram below illustrates the nature of banking systems at this time.
Basically the only technology used by most banks was the IBM mainframe hardware. Any software required to do something was written by the bank. Thus the bank wrote the business logic (“if balance less then 0 …” type processing); the screen formats (“put the balance in the top right hand corner”) and any security rules about what a user could or could not do. As the bank increased the number of applications that ran on the mainframe the amount of security processing software increased. Perhaps more importantly, as the number of users increased the admin required to keep lots of parallel files of security entitlements up to date increased exponentially.
Read further about the history of IT Security
Read also about the Likely Changes to IT Security in the Next 2 Years.