A monopsonist is the only employer in its labor market, so it faces the whole market supply curve. To hire one more worker it must raise the wage, and not just for the new hire: everyone already employed gets the higher wage too. That makes the marginal cost of another worker larger than the wage itself, which is why MCL lies above the supply curve and rises faster.
The firm hires up to the point where the marginal cost of labor equals what the worker is worth, Lm, and then reads off the supply curve the wage Wm it must pay to attract that many workers. A competitive labor market would instead settle where supply meets demand, at Lc and Wc. Both the wage and the number of jobs are lower under monopsony. The slider moves the demand for labor.