If you wish to be a famous photographer, take pictures of famous people.
When Karsh or any great photographer photographs famous people, we are instantly attracted to it because we "know" that person. And we know that person is famous. And we feel connected in some strange ways to famous people. Quirk of human nature.
Karsh was a studio photographer in Ottawa, the capital of Canada, and as a way to maintain the carriage trade of his city, he tracked down famous people coming through town and tried to arrange time to photograph them. He was a smart promoter and a great photographer, and he expanded his famous people photographs into a number of good books.
My theory of a great portrait is that is should reveal a person in some way, but the problem with doing it with famous people and especially actors, is that we have preconceived ideas due to he characters they play, so we really have no idea if it's a revealing portrait at all. I've said this before if you transpose an "unfamous"person into that exact shot, does it stand out. If it does its a great portrait. If not it isn't.
Probably the most defining portraits we see are actually done by photojournalists, migrant mother, Afghan lady, etc, partly because it takes us to another place and exposes the human condition.
But the bottom line is a great photograph is usually in the eye of the beholder. We put our own "stuff" in there, even when it may not have been in the essence of the soul of the subject.