Before you even ask the dreaded three-letter word ("WHY!?"), remember that as TR noted that day oh so long ago, DBE did well enough just in its foreign releases to justify the sequel, once we found out its budget was smaller than originally thought, and doubtless they think a sequel will do at least as well. So that's why.
The rest of the original interview with Marsters doesn't offer much detail on the film, though his desire to make a Piccolo-centric sequel is well-documented. He does talk a bit about the first Dragonball film and how they tried to "stay true to the essence of Dragon Ball." He also talks a bit about life after Buffy and the usual topics...and some unusual ones, like how it felt to kiss John Barrowman on Torchwood.
Let the bitching commence! To get you started-- what would you want a sequel to do, and what would you want a sequel NOT to do? And do you think Marsters is getting his wish for a film based on the Gohan-and-Piccolo-team-up arc?