Why is it that no US president before Lincoln had a beard or mustache, from Lincoln to Taft most had a beard?

For the same reason that so few people before your generation had tattoos all over their bodies. Fashion

Fashion. It was considered important in the old days. There were even a rule in the British Army in WWI that you could be court martial-ed for shaving your mustache if you were an officer. And why do you think that the generals around the time of the American Revolution wore those stupid, white powdered wigs? Looking good was a measure of an army's skill and importance, from the Renaissance to the Napoleonic Wars.

It fell out of fashion.