I'm only guessing here, but it may be similar to book publishing. How many times have you read a series and the first book was awesome, then each subsequent book got worse? That is because the author had an unlimited amount of time to write and polish the first book, making it as good as they were capable of before sending it out. But then they get their contract and they're expected to write x number of books in y amount of time. With less time to develop and write the story and more pressure to deliver, the quality suffers.

I suspect it's the same thing with games. Some are outright cashgrabs of course, Final Fantasy X-2 and the Compilation being obvious ones. Games also have an added dimension of risk, if the original developer should close or having a falling out with the publisher, or if someone else takes over developing. Sometimes they suffer from last minute changes imposed on them by said publisher. Final Fantasy XII, for example, had Vaan and Penelo added in at the last minute because they thought having an "old" main character (Basch) would be detrimental to the game, as the target audience would relate better to a young character. Bullshit of course, but that's how it was decided.

My problem with a lot of sequels is that because the original game never intended to have one, there just aren't enough loose ends to create a new story that makes sense in continuity/context terms. That's why you end up with poorly written, contrived and just plain weird plots.

~DragonHeart~