It’s usually the apps themselves doing weird things - Using undocumented APIs, expecting the system to be set up in a particular way, relying on bugs in the OS, etc. Windows tries, and actually emulates old bugs for popular apps so they continue to work, but it can’t be bug-compatible forever.
Apps/games that work on XP should mostly work on newer versions as long as you set them to run with Windows XP compatibility (in the settings of the EXE), but there’s definitely edge cases.
Can you install thid 25 year old program? (lemmy.ml)
So sad when it happens (lemmy.ml)