I doubt it is developer apathy. It smells like a management issue. iOS is where the money is. I don't know anyone at apple, but here's how I imagine it is there: Every OSX engineer probably has more bugs on their plate than can be fixed in one career, in addition to feature development tasks. The OSX team is severely understaffed both because it is not a priority at Apple, and because it is getting harder and harder…
Engineering is not organized by platform, but function. Apps that are cross-platform have engineers who move back and forth between macOS and iOS. Lots of code is also reused between the two. Both platforms use essentially the same tooling and languages (if anything, iOS development is more painful), though certainly iOS codebases tend to be better because they are newer. A fight for attention by a specific, resource…
The developmental move to agile deadline-driven feature-sprints under Craig Federighi.