Resource allocation. You can’t keep a massive team to maintain operating systems for different lineups. Even if they did, coordination would be a nightmare to give the same experience across different devices i.e, one having an issue for something which the other does not. Plus it’s a business, Samsung needs to make a profit for doing all this else it’s a moot point. So you first develop on the common codebase and then tackle device specific features. 50-70% of the developers would be the same who will work on these devices. So you need to stagger the releases simply because you don’t or in fact can’t have separate resources which the Apple can afford with just one lineup.