Could it be because they only have a handful of hardware variants to support, vs hundreds on Android? Presumably, the stack itself is somewhat sane on both sides, but the hardware's bugs, poorly specified or undefined behavior is constantly throwing wrenches in the machinery - Apple managed to fix most of them for the hardware they support, but Android has no chance as they have to support hundreds of different chips.
This is not specific to bluetooth or drivers, but I don't fully buy the thesis/deflection that Apple just has a much smaller set of hardware to support. I run a hackintosh system myself and it's more stable than Windows. I'm starting to think Apple just has stronger general QC. Windows is just as glitchy and crashy on Surface devices as anywhere else.
The counter point does apply to drivers, but it's really not just that.
