As well as the other reasons given here, the cost of conversion is much higher than you might otherwise think. Especially for banks, you have to prove that the

As well as the other reasons given here, the cost of conversion is much higher than you might otherwise think. Especially for banks, you have to prove that the new software meets with compliance regulations, not only for the financial regulators but also for the auditors (banking auditing is especially tough). Compliance does not mean just "meets all the unit tests", but also meets specifications like guaranteed uptime, stringent physical security (that alone could rule out many cloud services), tight audit trails, recoverable failures, incorruptible data, and so on. It opens up a whole can of worms that will dwarf the development cost of the software conversion alone. It's easy to see why many banks stick with their current working system.