I think the reason is simple. Khan came from india, where teacher quality is awful in many places. he want to offer good enough education to those who have awful education.
I'm from Finland, which tops in PISA rankings and what I've read about the reasons behind our good PISA performance, one is that teachers profession is still valued here, we have quality applicants and thus good teachers compared to many other countries.
Based on that experience, I'd say that Sal Khan is definitely above an average as a teacher. Anecdotally I'd say he is a very good teacher, but that's debatable.