strange, although you can see a road google maps can't calculate it maybe there is a road that is closed for winter, I heard from other people trying to plan a route across the alps in the winter, for their journey in the summer that it just wouldn't calculate it. In the summer google maps would once again calculate their route (as the pass was open again).
After looking at it again I think you can only take the ferry/Bus service and then only between May and September.
"Cape Wrath is inaccessible by direct road - to reach it you must take a passenger ferry (which operates from May to September) at Keodale. From the ferry end, a minibus runs to the Cape. When the weather closes in, the minibus stops running"