Why shouldn't something like skateboarding be in yet horse dancing and the (impressive but) really fucking creepy synchronised swimming should be?
I do get the thought that the Olympics should be the pinnacle so no football, golf etc but I guess it all comes down to what makes money and brings crowds in, and the more established most popular ones will do that, and when the thing already costs eleventy bajillion to host no one is going to want to turn them away.