why is the demand for professional labor generally high relative to the supply compared to the demand for unskilled or semi-skilled labor?

The demand for professional labor is high because now a days companies, have properly defined goals and agenda and tend to reach out to companies internationally.

Professional labor tends to be more useful and the cost incurred on them is less as compared to unskilled labor. The reason is that in this advanced world, the things that the professional labor already knows are the things the company would have to invest time and resources to teach the unskilled labor.

Therefore the difference in the demand remains.