It's entirely possible they liked your idea but didn't think you were a good fit for other reasons. Or they found someone else who is a better fit. Or someone e

It's entirely possible they liked your idea but didn't think you were a good fit for other reasons. Or they found someone else who is a better fit. Or someone else had the same idea. I don't think they should be obliged to hire you just because you had this one idea that helped them. It's possible they were not interviewing you in good faith, but just wanted free advice. But that's impossible to prove, and several other explanations that don't require bad faith are entirely plausible.