I strongly believe IGA puts as much effort into the storyline as he's capable of, but he's just not that good at it. Some people aren't meant to write stories. However, I think the storyline of Castlevania gets the job done, with little plottwists here and there. The Metroidvanias in particular had somewhat effective storytelling, albeit nothing we haven't seen before. And IGA tends to repeat himself quite a bit with the "good guy is actually bad", or "bad guy is actually good". To tell you the truth, it's only marginally more sophisticated than Mega Man (which is meant to be silly, which I doubt Castlevania is)