Well, to be slightly more specific, it's not 24" wheels that make the bike feel streety, it's the geometry of the frame (wow, i'm clever). It just happens to be that riders expect a 24" bike to have a streety feel and so manufacturers make the frames short, with a low rise and longish rear end. I'm pretty sure that you can make a 24" bike feel just like a proper trials bike by giving it a wheelbase around 1065 and a ~35mm rise. But as the 24" frame market seems to consist mostly of riders who are into street-trials, it'll take a bit of time before we see a trials-specific 24" frame.