Because the discrepancy exists and shows consistency. This gap needs to be called something, you can't just pretend it's not real. But the underlying model seems fine and all alternatives are broken.

So we came up with an idea that fills the gap in the current model and allows us to continue predictive science until we find out more.

People get so absurdly touchy about Dark Matter just because it's named and "weird". It's just a placeholder value.