the average cabby can't buy a medalion, he therefore has to pay a fee to someone who owns a car with a medalion to rent the car for a period of time (+ probably paying the owner a percentage of his take). As opposed to using his own car and keeping everything that uber lets him to keep which might be less than what the medalion owner would have taken. hence, increase in money into pocket for the uber/lyft driver.

with that said, this is all supposition, i just find it a reasonable argument.