Free advertising/buyer base, basically.
The people who like the previous game will, at the very least, pay far more attention to the media buildup and marketing of the next installment. I'd suspect someone who has does want to see more of a franchise would be more likely to see the positive factors of marketing/reviews, or at least weight them more.
From a marketing perspective, that's a built-in audience that a new IP is at a massive disadvantage against.
OlasDAlmighty said:
And really we the consumers aren't totally without blame, we buy sequels more than new IPs, so we give them a reason to make them.
Exactly this; you want more originals, then me, you, and preferably everyone you can get to listen, and hopefully sooner than later every other consumer need to ruthlessly shake off brand loyalty and pay a lot more attention and sales to releases that are trying new IP's. That's not to say sequels should be considered inherently abominable, after all something made the franchise-starter popular enough to greenlight a sequel, but buying based on just on faith in a brand name alone is a habit that I'd really like to see gamers (and consumers of every product, really) shirk sometime fast. Until then, Sequels will mostly just be seen as an easy phone-in investment that generates superior returns, and that's not changing unless both a rising demand for new content and a backlash against franchises that outweighs the built-in audience of sequals balances which has the advantage.
Fortunately the big publishers seem to be doing a great job wearing out their welcome, and just maybe some big names will bomb and the AAA industry will be put in a lethal 'adapt or die' scenario sometime in the near-future
...
I can hope at least, can't I?