In any case. The reasoning is like this:
1) Death note = horrendously popular. Now, this isn't USA where if something's popular you make spinoffs until it becomes an unprofitable internet meme, but Japanese in their Samurai ways probably still see the potential cash just sitting there and would like the author to continue it.
2) Death Gods and their origins haven't been even touched on in the series hardly. Who's to say how they're created? Who's to say that what Ryuk said about the user of a death note being destroyed (like, spiritually, no heaven no hell no nothing) was correct? It's not like they're not capable of lying, Rem lied lots when it was a good situation to do so. Perhaps the user of a death note can become a death god somehow?
3) A shinigami which - to many - appears to bear resemblance to Light is shown at the end (?) of the soon-to-be-subbed 3-hour Death Note special that aired 2 weeks ago.
4) Everyone REALLY WANTS A SEQUEL, as long as it's good. Everyone. Even the ones that say "The second arc wasn't very good", would jump at the idea of a new season of quality comparable to the first arc of the first season.
5) This was earlier concept art so it's debatable whether or not it means anything, but I'd like to hope it does. Appears to be either just a Light shinigami or a fusion of Light and other characters. It's got the tanned skin of light with the pale skin of L (?), etc. Mikami-ish hair. Ryuk's blue skin...

6) There's some more I think, but I don't remember it so well...