Just what good would a wheel have been in the breaks, bottoms and even on the prairies? Something else to break, labor intensive to create, and no roads (more labor intensive) to use it on.
For their environment, the travois did just fine. None of the mountain men or French trappers (nor even Lewis and Clark) were using a lot of wheels, either.
If it ain't broke, don't fix it.
Seems like several hundred thousand or more immigrants crossed those rough terrains on any of a dozen trails to CA, OR, and various other now states in wheeled wagons just after the fur trapping period phased out. Then, they built this country.