1. They showed the paintings to people who are probably not interested in this sort of art and thus don't know what the buyer would be looking for.

2. Tying in with 1, the value of any artwork is as much about the buyer's perception of the artist as it is the quality of the artwork. Almost all artists, regardless of technical ability, rely more on their ability to generate a personality cult around themselves than they do their skills. The average buyer wants to be able to say they have a <insert name of famous painter>, not that they have an exquisite piece by someone nobody has heard of. Most buyers are trend-followers, not trend-setters.

3. Bandwagons. Nothing is more important for becoming a popular artist than producing the right picture at the right time. You have probably seen and been amazed by how fast fanart of the hot new videogame/anime character gets pumped out - there can be a hundred stunning images from some of the most popular artists online within hours of a new character being announced or becoming popular. This happens because striking while the iron is hot is paramount if an artist wants to get the most eyes on their work. It's no good producing an amazing picture of something people aren't currently interested in. It's far, far better for you produce a merely 'good' picture of what people are clamoring to see at that moment. ...I can't help but notice that most of the female artists here come decades, if not a lifetime after the male artists they're being compared to.

4. They cherry-picked the paintings they showed in the survey.

5. If women are better painters it is because their fathers don't beat the shit out of them for saying they want to be painters.

source: I get paid to draw porn and it's exactly the fucking same.