Freeway driving is much, much easier than in-town roads - there's been successful autonomous freeway driving since the 90s.
When you drive on the freeway, everyone's going the same way (or at least you'd hope), the configuration of the road is well known, lines are consistent, etc. It's all the little things that need to be accounted for - opposing traffic, pedestrians, parked cars, random obstacles, and any number of other road, traffic and obstacle configurations that exist, that make consistently and safely driving elsewhere difficult for autonomous systems.