Pitch Black was uninspiring, unfrightening and not very gruesome. It has poor computer-drawn beasties and only shells of characters. Some unresolved background for Riddick is left unexplained and covered in Chronicles, but all of it is passing, none of it central to anything.

Riddick is also a big wuss compared to his behavior in Chronicles, but there are several years of development for him in between.

Putting all of it together, I have no idea how it did well enough to spawn a sequel.