Question

Why is the equilibrium wage higher for doctors than for convenience store clerks?

Answer 1

one of the reasons might be that there r less doctors (less offer of labor in that market) than people with the (very low) required skills to be a store clerk.
other reason could be that in order to get the required qualifications to be a doctor u need to go to a (good) univesity and usually that is very expensive. so MD graduates would not accept the low equilibrium wage of a store clerk