mac OS is built by one major player and they have a clear direction of where they want to take this. Linux, in general, is built by communities of people who may or may not have diverging opinions, priorities and interests. While this gives you a lot diversity, it can be quite problematic.
The financial aspect is also important. elementary LLC itself, has only 2 full-time employees, while Apple is sitting on a pile of cash. Yes, elementary builds on a solid open-source base [1] built by thousands of developers world-wide, some volunteers, some employees, but it's still a significant difference.
Having a smaller scale, it means elementary can be more agile in its changes. This also means that there is smaller testing base, which means there will be more bugs. The nice part is that, when you find a bug, you can help fix it [2].