Bicycling/Driving license for Mountain bikes.
What are the reasons for driving licence not required to drive a Mountain bike?.
Non-motorized vehicles, including mountain bikes, are much less potentially dangerous than motor vehicles, both for the rider, for bystanders, for property, or for other vehicles. Non-motorized vehicles are also much less likely to be used for crimes. Therefore, most governments do not see a need for riders to obtain a license.
Also, setting up a license system would involve setting up another government bureaucracy or division, which would be expensive. Employees would have to be hired to test each rider and print out driving licenses. It's just not worth the effort and expense unless bicycles start to injure large numbers of people, or become frequently used for crimes or terrorism.
The United States is trying to promote and encourage bicycling for fitness and to reduce traffic congestion. Subjecting riders to a licensing system would discourage rather than encourage bicycle riding.