However, let's go again; the reason why soccer hasn't broken through in America (or Canada for that matter) is because the sport- market is saturated. It is saturated because soccer "wasn't there" when professional sports began.
Why wasn't soccer there then? I don't really know for sure but because it wasn't there Americans had since developed their own sports and therefore to them soccer has to this day been as a foreign sport.
That's not accurate. Soccer is not a "relatively new" sport to North America. It predates football, hockey, and basketball in the United States. IN fact, soccer was only second to baseball in popularity, during the early part of the 20th century. The United States did very well in the first World Cup (1930). The decline of soccer in the United States, was the result of the Great Depression, and the dissolution of the American Soccer League (ASL) in 1933.
I read this on Wikipedia today for the first time, about the demise of the ASL, and it has an extremely interesting take on why Americans turned their backs to soccer in the 1930's:
American Soccer League (1921–33) - Wikipedia
"However, the Soccer Wars had permanently crippled the ASL and it collapsed at the end of the 1933 spring season. Ironically, while USFA and FIFA "won" the wars and established their pre-eminence over the ASL, the spectacle of a U.S. athletic association conspiring with a European organization to undermine a U.S. athletic league alienated many U.S. sports fans by creating an image of soccer as a sport controlled by foreigners. These fans turned their backs on soccer, relegating the sport to the position of a minor league, ethnic-based sport for decades to come."
In other words, FIFA is to blame, for the decline in popularity of the sport in America:
The Golden Era, 1921-1933
The sport of soccer was relatively obscure until the emergence of the North American Soccer League (NASL), in 1968, and the signing of Brazilian superstar Pele by the NY Cosmos during the 70s, that helped the sport regain popularity. Soccer still has struggled to become a mainstream American sport, but it's popularity started to grow once the US won the right to host the World Cup in 1994.
Having said that, there is currently no soccer league in North America that can compete with the NHL.