Because all things must come to an end.
Besides, as fun and compelling as Blood+ is, we wouldn't want it to go on forever. Things get old and annoying that way. Take Naruto for example. Or Inuyasha. They're fun at first, but after awhile they get annoying, and the viewer gets tired of watching the same old thing. The motto is keep it short and simple. One reason Blood+ is such a good anime is because it is so short, just like Fullmetal Alchemist.
I hope I helped.