It tells you something about the power of Samsung in the Android world that they can just say "no, we're not doing that" and Google back down.

That said, why don't Samsung do it? I mean I know their ROMs are huge, but it's not as if their flagships lack storage or as if they've been shy about allocating space to the system (I don't know whether they are more sensible now, but I remember days when higher capacity models had larger system partitions even though the ROM/firmware do not scale with storage at all). Why are they so set against adopting the A/B partition scheme?

 

Why can't they adjust if it's warranted for functionality? Or is there an edge if they are unique amongst competitors?

 

Unique in not offering a feature that makes updates more painless but has no effect on functionality otherwise isn't exactly a competitive edge, so I think we can dismiss that motivation.

As for why they can't deviate, well clearly (because of their market power) they can. If you are asking why Google mandated it, there are an awful lot of things that Google mandate and that manufacturers have to do, and a number of those can fairly be classified as anti-competitive but manufacturers go along with them (and many, including Samsung, have never publicly voiced any qualm). This one simplifies updates and doesn't stop manufacturers doing anything else, so has no downside to the user and no obvious one to the manufacturer. So the question of why Samsung choice this hill to fight on remains valid: is this just some massive corporate "we can't be bothered", or is there something peculiar about Samsung's workflow that this conflicts with?