There are so many reasons that jazz is unpopular. It goes far beyond a single reason like "Americans prefer visual art". As a big fan of jazz and lots of other styles of music, here a few of those reasons I've seen in action:

1. Jazz has a long tradition of taking "standards" (popular songs largely from the 1930s-1950s era) and reinterpreting them with ever more complex harmonic arrangements. This has been going on for generations and we are several iterations down the rabbit hole of re-interpreting the same songs into ever more avant-garde versions. This makes it incredibly hard to get into jazz as a new listener because you wouldn't be familiar with those original songs or care about how they've been re-worked.

2. Jazz musicians and fans tend to be very protective of their art and compete with each other to show off the depth of their knowledge. They can really put off anyone trying to get into jazz on their own. Musical neckbeards abound.

3. Jazz originally became popular by taking the most popular songs of the day and re-working them to sound daring and original by using more complex harmonies (think Coltrane's 'My Favorite Things'). But many jazz musicians are still playing the same 70-year-old songs that modern audiences don't care about. It has no relevance to a modern audience.

4. 80 years ago, Jazz was cool because it was mashing up pop music in new ways and it sounded threatening to old people. It was risky - the rap of its day. Now it is the least cool thing that could possibly exist and the audiences are nearly only old people.

5. Jazz is all about seeing how far you can push harmonic complexity. It's a breath of fresh air for people who are bored with hearing basic major and minor chord harmony over and over. But a lot of what's popular right now with kids (i.e. soundcloud rappers) is very simplistic harmonically. There's a lot of atonal-mumble-rapping over beats (which is what sounds dangerous and cool in 2019). What jazz is bringing to the table just isn't what young audiences are looking for right now.

6. So much music that people hear that is labeled as "jazz" is truly terrible muzak. That puts off possibly interested people from even trying out jazz.

7. Even the word "jazz" sounds dorky in 2019. Think "jazz hands."

8. Mainstream jazz since 1975 or so has produced some of the most navel-gazing, unlistenable theory-music imaginable. The greatest timeless jazz music, like tracks by Miles Davis and Coltrane have 70-80 million plays on Spotify. But no one outside of Jazz fanboys want to listen to whatever Pat Metheny is playing on his 42-string guitar-harp. It just sounds dorky.

All that being said, I think there is tons of great jazz and jazz-adjacent music being made now. It just isn't always branded as "jazz". Here are some things to check out - plenty of young people killing it in the jazz scene:

Julian Lage (former child prodigy / jazz guitar virtuoso) - https://www.youtube.com/watch?v=q5ggv-5s4bs

Jacob Collier (former child prodigy / everything virtuoso) - https://www.youtube.com/watch?v=IlFD298wTOM or https://www.youtube.com/watch?v=VPLCk-FTVvw

Vulfpeck (funk / just having fun) - https://www.youtube.com/watch?v=r4G0nbpLySI

Jon Baptiste (maybe you know him from the Cobert Late Show) - https://www.youtube.com/watch?v=M3qTwHRECmM

Punch Brothers (more folk, but here playing with Jon Baptiste) - https://youtu.be/alW_ZSMw7sM?t=70

Cory Henry (Keyboard virtuoso) - https://www.youtube.com/watch?v=I8j7XJxew7s

There are tons of others and lots of overlap with hip hop - check out people like Thundercat.