I think some people don't see the point?
My mother was that way - for most things she was overly practical. "What's the point?" and "What is it good for?" were common refrains when I or my siblings asked to do an activity. Even organized sports were questioned - I liked doing gymnastics and dance, but after a bit my mother asked why I kept signing up for classes since I "wasn't good at it." As in, I wasn't winning, so why even do it anymore.
I think many parents see the point of sports, and sometimes music or an art class, even if their kid isn't particularly good at it. But with academic competitions, the peripheral benefits are harder for most parents to understand. Also, academic competitions come with less bragging rights. I mean, the social cachet is less, but there's also the problem that (in the younger grades) there's no progression of my kid won County meet (and people saw it) and my kid won State meet (and people watched) and my kid went to Nationals (and it was a big deal) and got a bunch of huge trophies, Do you see them? No, it's you get a certificate and maybe a ribbon in the mail and an invitation to try again next year. Where's the thrill in that?
I know it's not the fault of competition organizers who run on small funds and would never be able to charge the same fees as any of the "national" dance competitions. It's also not the kids' fault, who are probably pleased with their small recognition and look forward to the next year. Call me cynical, but I think if there was a math competition set up like a chess meet, where kids faced off with each other one on one and there were rankings and lots of huge trophies, parents would be more in favor of it.
Also, I think you also have the problem of moms saying "Math? Eww." Unless they have a huge personal incentive to think beyond their immediate emotional reaction, they won't.
And....that's my social theorizing for the day.
And yes, my mother was hard to live with.