I guess because a big road like that takes a while to build? Although I don't know enough about building roads to know how long it should take.
My guess is they're still scrutinising the route to see where else they can stick some round-abouts, traffic lights, 20 mph zones and zebra crossings to make the road as much of a PITA in the ass to negotiate as possible. See every other fucking road in Aberdeen and Aberdeenshire as reference.