Hasn't nVidia had a kernel memory manager longest of all the drivers? You'd think it's so well optimized they wouldn't care what their competitors do.
On the other hand, maybe VIA would... their drivers on any OS make fglrx look like NASA software by comparison.
The closed source memory manager is designed around a completely different driver stack, all parts of the stack (2D, 3D, etc.) have have to be substantially rewritten to use it.