It's the price they pay for being large, bureaucratic organisations.
One main reason is because they continuously have to support possible legacy systems of their bigger customers. Financial organisations aren't the slowest ones to upgrade. Many non-IT companies are even slower, simply because the IT-branch isn't their line of business, so they're saving every penny on IT stuff.
Also, older software tends to have proven itself. They could keep up and invest in the latest techniques all the time, but who will guarantee them the new technique will become successful? For example, in 1994 a tool called Gupta SQLWindows was introduced as a new development product. It was supposed to become a great RAD development system, ready to compete with Visual Basic. Unfortunately, it also had to compete with another tool called PowerBuilder and both had to compete against Borland Delphi.
Back then I worked for a company that had to decide between SQLWindows, Powerbuilder and Delphi. I advised them to use Delphi, since it seemed to offer the most possibilities but unfortunately, the CEO (who also did some development work) only understood BASIC and Delphi used Pascal as it's root language. Powerbuilder just appeared to be most similar to BASIC so he chose for Powerbuilder and I moved to a company that did use Delphi. Three years later, my old employer was dead-broke and the new company I worked for received a nice order from a financial institute to build some software for them! (In Delphi!!) This bank had just waited to see which technique would be the most successful and then just outsource their development work to a company that used that technique.
And basically, that's what many financial institutes do. They don't do much development themselves but prefer to outsource those projects. It's more secure for them to outsource projects simply because it allows them to cancel those projects without getting stuck with a bunch of software developers who need another job...
Of course, financial organisations also like the principle: "If it ain't broke, don't fix it." You can find quite a few antique systems which are still in use to keep track of invoices and other data. Why? Because those systems have proven themselves to be reliable. So why upgrade to a new system? That could only introduce more errors to the system.