Is there any reason why that approach wouldn't work on X? I read a few of the articles on it and seems like it could be implemented in X if somebody spent the time writing it.
People have spent the time writing it, but it's still too buggy to be deployed. Somehow, it even has a tendency to actually corrupt the SD card. I'm not a GPU driver developer, so take this all with a grain of salt, but my understanding is that X is harder to support because it is such an unwieldy behemoth. Not only do you have to write kernel modules, but you have to write a user-space driver for X (known as a Device Dependent X or DDX driver).
With Wayland, the entire concept of a DDX driver disappears. There is a compositor backend that knows how to talk to the appropriate kernel module, and everything is happy. It's probably this reduced complexity that has made it so much easier to develop an accelerated user interface with Wayland than X.
This is not to say that Wayland isn't an improvement, but with the RPi drivers it doesn't take much.