Probably the parent commenter has much more insider info than all of us since he's currently at NVIDIA...

From what I understand, PhysX has been built primarily as a physics engine middleware for games. So when folks at NVIDIA tried to extend this engine to robotics (for IsaacSim/IsaacLab) it seems they've faced lots of challenges (mainly with subpar multi-env performance and inaccurate solver, but also lots of technical debt over the years). So changing the internal engine to a more robotics-oriented one (Mujoco-warp) doesn't seem far-fetched. Nowadays for game engine development there are much better middleware CPU-based physics engines available (mainly Jolt Physics) - and GPU physics in games aren't that popular anymore due to pragmatic reasons (the GPU -> CPU roundtrip defeats the whole purpose of better performance)