Because one Super Mario Galaxy game wasn't enough to contain all the game's fantastic ideas.

Sequels can go into one of three different categories - completely unrelated to their predecessor to try something new, being an evolution of said predecessor that expands upon its ideas, or made quickly to cash in on something's popularity.

Do I REALLY need to say which one this game is?

...It's the second. If you say it's the third then I'll sneeze in your mouth.