The thing is that nVidia could use a-sync and the new option in the DP standard to make a variable framerate solution but AMD can't use the G-sync sollution.
And I as a customer want a product that don't lock me to one GPU vendor.
They theoretically could, sure. But they won't, because they feel they have a better solution.
Variable refresh will lock your GPU choice, period. Should it be that way? Maybe, maybe not. But it will, just as surely as your choice of motherboard defines what processors fit in it.