Complex sentence: The MLS Cup's roots trace back to the foundation of Major League Soccer, when the league decided to have a championship format similar to its contemporary North American sports leagues by having the season culminate in a postseason tournament to determine the league champion.
Split into simpler sentences: The MLS Cup's roots trace back to the foundation of Major League Soccer, when the structural format was being assembled. The league decided to have a similar setup to its contemporary North American sports leagues, by creating a postseason tournament to culminate the regular season, which would be the determining factor in crowning the league champion.