The three most popular sports in America, Baseball, Basketball and Football, are almost exclusively American. Although they are of course played and watched in other countries, primarily, they are played and followed by Americans, in America. The global numbers are far smaller than many other, more worldwide sports.
Soccer is a worldwide sport. It is played on every continent, one of the only sports in the world to do so, is watched by a truly global audience, and regularly boasts the highest-earning players and most followed organisations.
Major League Soccer, being based in America and Canada but partaking in a global game, is attempting to span the gap between the two.