That's actually a great question. The movie industry is more of a get-rich-quick industry. They play the game of figuring out how to make the most money out of a concept that they can whittle into a blockbuster. If they make a bad sequel, as in they don't put forward much money into making the second one, they're guaranteed a considerable revenue from people who saw the first movie. It's essentially their way of getting the best return on their investment.
Video games are a new enough industry that there are still developers who are genuinely concerned about game quality. It costs much less to make a video game than it does to make a movie (generally), so the developers are a little more free to add quality as a series goes on and are a little more inclined to put the money earned through sales of the game towards improving the quality of the next installment.