I'm sure for Apple it's QC, build process, documentation, examples, and constraints around building new things--maybe sharing stuff between iOS or preparing for a transition to ARM. Who knows? Apple tends to support the old OS for a few years, so while it does suck, it should be manageable.
It drives me nuts that installers and license managers are old and crufty. I do understand why. A few times now I've encountered situations where the installer cannot run even though the software it installs is fine.
I understand why it benefits Apple. I think the idea that it benefits users by forcing app developers to write better apps is a load of nonsense.
Separately, I feel like Apple should deal with it. They're one of the richest companies in the world and they can afford to maintain extra copies of libraries, possibly indefinitely. No one is asking them to go full-on Microsoft and explicitly test against old 3rd party apps. But, Apple clearly doesn't care, and I can't do much.
It drives me nuts that installers and license managers are old and crufty. I do understand why. A few times now I've encountered situations where the installer cannot run even though the software it installs is fine.