This is the answer you are looking for:

the sacrifice to their personal and social life is too high.

https://news.ycombinator.com/item?id=20853670

It's big fish in a small pond syndrome. If you have multiple years of college and your interests are fairly intellectual, you are going to be hard pressed to find like-minded friends, etc, in a rural area treating farm hands and the like. It's also tough being, for example, the only doctor in an area because then everyone is your patient and you have an inherent conflict of interest when trying to make friends.

“Money isn’t everything.”

That’s the short answer.

For instance, what about your husband/wife? They just put their career on hold to go live in the boonies for 5 or 10 years? That sort of thing will put an enormous strain on your relationship.

The pay is better, but not sufficiently better to offset benefits of urban life. Offer $5M per year, and it won't be challenging to get physicians to work in rural areas. So the problem is merely that there isn't enough funding to incentivize doctors to work in rural areas.

Not the person you wanted a response from, but money isn't everything.