Why does a safe need an operating system?
Because it is computerized and does more than control a lock. When was the last time you saw a computer without any sort of operating system?
And then why for heavens sake has it to be a desktop operating system?
Because that's what most people know how to write software for. Not saying it was a good choice but I understand why they did it.
It's not like there are especially hardened OSses out there for embedded devices.
It's not an embedded device. It runs a pretty much bog standard PC. I've actually worked on some of the hardware in these in my day job a while back on a project. (No I had nothing to do with the design or the implementation of them nor do I have any relationship with Brinks and no the project had nothing to do with hacking them)
And after having such a terrible design idea, why have it implemented by a moron using an out of date, unsupported, and buggy OS?
Because it wasn't out of date or unsupported when they designed the safes. These aren't a brand new design. Again, not saying it was a good choice but it made sense (sort of) at the time.