If the clothes are so unappealing, why does the “equestrian look” roll around with some regularity in the fashion world? There are probably still people around like my mother, who started riding primarily because she thought the clothes were so pretty.
When people watch football, they have an understanding of what’s involved because they played football in high school or college, or their child/ren played. Golf is pretty accessible - public golf courses, even Putt Putt gives you an inkling of how challenging it can be. Lots of gymnastics lessons being taught. Figure skating is pretty common in may parts of the country. Kids play ice hockey and there are teams all over the country. Track and field - people have done it in school and everyone knows basically how it works. Etc. I know that participation at the highest levels of these sports can be expensive, but not as expensive as riding.
When I was a kid, lots of girls in my school took riding lessons. There were riding schools all over town. Now, where I grew up, as in many places, to be able to ride you have to have your own horse, which is totally out of reach of many families. Rich people stuff, elitist. Not relatable. We all lived and breathed anything with horses in it - racing, show jumping, whatever. But how can kids get interested in horses if they’ve never even touched one? How does that help grow the audience for show jumping? It’s easier to just watch basketball.
Add to that the growing bunch of people who, never having had a chance to be around horses, think everything we do with horses is cruel and the poor beasts should only be free and galloping across the plains. I don’t know, there are problems, but I don’t think spectators not liking jumper riders’ clothes is really one of the main ones.