There could be several reasons for that. One is that artists who make 'reasonably good' money are the ones who have their name/brand out. So people tend to pay more to them over lesser known good artists.
What are some other reasons in your opinion?
1. They like it and want to put it on their walls. (This is most art, and artists selling to this group need an additional income.)
2. They're seeking the validation from their peers that comes from showing you have "good taste". (Artists selling to this group can make a moderate living but they won't get rich.)
3. They're investing. (Artists selling to this group can get to be millionaires.)
She has decades of craft and decades of knowledge about what will sell and decades of rolodex. It's a business. She still does art for its own sake and it is way more awesome than what pays the bills...but what pays the bills is what pays the bills.
One of the things about Nan is that you wouldn't know she's an artist from meeting her. No black turtle neck, no angst, just the woman who grew up from the child and went to the grocery store to buy milk when the kids drank it all.
So what is the reason that most artists don't make good money? It's hard work and luck and perseverance, like many other occupations and not like programming is today in that there is not a lot of corporate demand.