Well, the game sells Wii consoles worldwide, sells well in Japan, is E3 2006's best sports game, is TIME Magazine's game of the year...
Doing so well offline, you'd have to admit, if you ran Nintendo, and you expected such a response, you'd save online functionality and other features for future games too.
Nintendo's always tried to avoid cookie-cutter sequels...even the most "guilty" franchise, Mario Party, tried/tries to add things like day/night, microphone, eight-player, and all-new games, etc. If there was a fully-fleshed out sports game right from the start, they'd have less to add to another one.