Because it's cheaper.
That depends on what you count. It might be cheaper in the short term to keep patching together an increasingly obsolete and aging system than to develop a new modern one but if you look at the longer term the higher maintenance costs add up. On top of this there is a lost opportunity cost: you could miss out on new, profitable technologies and methods if you have an older, less flexible system which is why banking innovation seems to be increasingly happening in companies outside the sector e.g. things like Apple pay.