Because they liked DMC5 and aren't invested enough in the storyline of the series to want to give up the chance to have more of that gameplay for the sake of respecting the story's natural endpoint. There's also the option to just start a new storyline, in which case it might not be numbered as a direct sequel. Either way, the point is that people want to play more games like the good game they liked.
This is my signature. It exists to keep people from skipping the last line of my posts.