I would think it's because originally this was top-down pseudo 2.5D and there, these animations really made sense and is how these game were done for eons. However for side-scrollers I agree that the fluidity and diversity of the animations should have taken precedence because you are constantly seeing them. And I think that whatever one sees 99% of the time should be of the highest possible quality.
That said, for this kind of game the animations should either have been /drawn sprite based or fully 3D because that would have allowed IK and rag-dolls. I guess so would have the 2D version of that (each limb separate with a layering function and as bone attachments)