For the benefit of people that haven't had the dis/pleasure of working in a really big company, the large banks / govy places often have a ton of dependencies and at (least) one place I worked at had a custom tailored build of Windows with features they need.

They're also heavily regulated and if they can't show evidence that they've done all the required audit checks to ensure they're in line then the repercussions make millions look like chump change (think an investment bank being banned from trading on certain desks).

Depending on their priorities and at that size this might be the most economical move.

It's worse in government. I knew a very important Australian government department that used four levels of test for a service management system.

It's not just about upgrading the OS - it's also about upgrading the software built for that OS, which in some cases requires a complete rebuild.