Because that is what the game industry is all about now. you simply can NOT make a single game that stands on it's own, there is no such thing. Every single new IP is meant to have sequels. This actually applies to the movie industry as well. Sequels make the most money. I'm blown away that AVATAR has made as much money as it did without being a sequel... imagine how much THAT movies sequel is going to make... because it's obviously going to get one. just wait.
I'm sure they'll find a way as long as Dante's Inferno sold reasonably well, which from what I can tell, it has. They'll probably come up with a different character to go into hell, or some way for Dante to go back, I haven't played any more than the demo, so I don't know what would make sense for a sequel, but if the money is there there's always a way.