I could go on for 3 paragraphs, but I'll give u the simple answer: Money
If u have a huge fanbase for a game, why keep continuing patching or expansions if u could make another game with the same name, knowing the current fan base would buy it in 2 seconds?
Btw, ur way off about Resistance. Is that really the ONLY thing u noticed? How about the fact u can't use more than 2 weapons, or have a regenerative life instead of a health bar (similar to modern shooters)? You think they could just magically patch in more players..? Not to mention a whole new cooperative mode, character development, a whole new story, etc. Maybe the modes, story, and weapons could fit in an expansion, but as for the gameplay; if ur gonna change that much of the core gameplay, why not make a new game? Yeah..
Also,
Sequel - a literary work, movie, etc., that is complete in itself but continues the narrative of a preceding work.