Basically, people will participate in competitions and events where they have an opportunity to get a reward for just spending their time and they don't need to spend anything from their pocket, especially if they are participating in something that they don't have a lot of knowledge about, in this case, it's horse racing.
As far as I am concerned there is actually a big market for horse racing, but we do not see it much around here because people who are dedicated to it are within a category I would call "old school", in terms of gambling.
It is one of those sports or activities you need to be very into and investigate in order to bet or partake in it. It is like other less popular sports, like golf or even chess, they have not have so many fans as basketball, football or baseball have, but their fan-base is very dedicated for them to continue to survive the passing of the time. Here in my country is possible to bet on horses but it is not something I would expect a 20 year old gambler to do, on the other hand, my grand father used to.