Nvidia didn't implement implicit sync because it was stupid and also didn't really solve anything, it still had performance issues.
The real problem with explicit sync wasn't Nvidia, it was the fact everything and everybody has to implement it. This problem was worse under a stack like Wayland where every piece has to reinvent the wheel.
The missing piece of the puzzle wasn't one piece, it was all of them: explicit sync had to be implemented in the kernel, and in drivers, and in graphical libraries, and in compositors, and in apps and so on.
Nvidia released it after it was stable in the kernel.
They don't care about Wayland or any other userland applications except their own. They don't have to schedule their development around Wayland, why would they? It's an emerging stack that's not yet in use across all the Linux desktop, which is like 1% of their user base anyway.