Here are the results (also in original post)
Despite getting a few more votes in the poll than usual, two answers actually got no votes, "It was a better game than Forza 1" and "Better advertising/promotion". As you can see the "bundles" option was by far the most popular, with "franchise gaining momentum" in a distant second.
My take: I'm not that familiar with Forza so I will keep this shorter than ususal. As a number of posters in the thread pointed out, sales of Forza 2 really shot up after the bundle was introduced, so that is obviously a big factor. That said the game had already surpassed the sales of Forza 1 by a decent margin by the time the bundle was introduced, so I do think the franchise is picking up momentum. I also think the lack of competition may have played a role, both on the console and in the genre. It was released fairly early on in the 360's life, so a lot of people may have picked it up since not that many other big name games had been released yet. Also we all know how huge the Gran Turismo series is, but before Forza there wasn't much competition in the racing sim genre, and even less competition that was of comparable quality. So I guess the explanation of why Forza 2 increased is not that interesting, but what will be interesting to see is if people who got Forza 2 through bundles, or because of the lack of competition will buy Forza 3 when it comes out. What do you all think?