Because of the money involved, nearly everyone in the USA eliminates basketball, baseball, and football as a possibility (usually in high school theyll know they arent good enough in the big 3 sports) before they start to try fighting, and same for the rest of the world in soccer and basketball.
If we ever reached the point where MMA somehow paid about the same as the NFL, MLB, international soccer, and the NBA for, say the 50th-best athlete in each sport, then priorities would shift and people may be more willing to choose MMA over the other sports, but right now you'd be an idiot to choose fighting if you had other options. Hell, even if the 50th-best UFC fighter got paid a third of what the equivalent in the other sports make we'd see movement.