Minecraft can do it because they aren't adding new features, they are adding content based on a solid foundation, they aren't changing the core of the game anymore, just adding onto it. When they changed to the Anvil file format for saves, they had to hand write a converter to keep old saves compatible. They only had to do that once, but for Rimworld at this stage of development, every update would require a new converter written.
With Rimworld the very base of the game is changing every update, how parts interact with each other, how the saves are done, features of items, performance tweaks, etc, which is why they are not compatible.
With Rimworld the very base of the game is changing every update, how parts interact with each other, how the saves are done, features of items, performance tweaks, etc, which is why they are not compatible.