The only assumption we can make is that there is not heavy Waze usage in your area yet. When we recommend that people drive the slower route so Waze routing servers can learn, that is assuming that there isn't anyone else on the road using Waze driving that route. When there is heavy Waze usage in an area, Waze will get that data from other drivers and learn that it's slower without you having to drive it.
What city are you in?
I am in Hanover, PA, and I drive to York, PA. I agree there are probably not many other Wazers in the area at this time. I don't see any other driver icons on the route I take either way.