A lot of banks & building societies built mainframe systems to run mortgage businesses in 70s. They're still running on them, and don't want to bear the cost of transition. When the housing market is buoyant they transact and make money, so a system overhaul isn't a priority. And when times are bad, there's no money for that kind of project. As for the software vendors serving this sector, that's another story...
This has been my experience after 7 years on the IT side of the financial services industry. I worked at one of the top 10 commercial banks in the US and within the last 3 years they abandoned a multi-year migration project to a real time infrastructure. Many huge banks still run Tandem systems.
Traditional banks are very risk averse and have large investments in older tech.
It's a brave company who chooses the ground-up re-write of a core system in the banking world.