It requires skill, which automatically rules out 99.9% of this forum. No offence.
Though your right by what you said when it comes to it physical gambling, those that will bet on horse riding have to be experienced in it, however when we consider the online gambling on horse riding as well which requires no experience, we don't also have much fans into this because it's not one of the present century games gamblers had love to play, most of the horse riding bets played now are virtual bets and not live match from the casinos and gaming stations, we can notice that there are many factors that affects this category of gambling to be less accepted by most gamblers.
If we base on actual game then we can say that it will need more practice and more patience to do that as we all know that how hard to ride a horse and as we think that it's more harder if it is on racing cause you need to command the horse to run faster as he can in order to win the competition. Anyway in online gambling there's a machine you can bet on horse racing actually here in my country before there are many machines who support horse racing but it's illegal once get caught then years of imprisonment will weaving .