It takes an ENORMOUS amount of work to go from an idea to a company that's in the black and producing. In addition to the idea, there are lots of stars that have to align in order to make that idea a successful reality (funding, team, development, sales, support, etc...) And, though the idea is critical it is very much over-rated, lots of folks believe that the idea is sufficient to the success of an endeavor, and that the implementation and execution are just details (which obviously couldn't be farther from the truth, the execution IS the thing, the idea merely the spark that starts what you hope will be a fire).
Anyone who's been down this road before and realizes the commitment that's required will be rightfully cautious about vetting the opportunity before jumping in. Likely they have other options already and have to weigh your against the others. It's sort of like getting married and posting an add looking for a spouse with a short description of yourself and then expecting someone to say "I do" in response. You may find someone willing, but is that really the person you want?
A great source of co-founders if you're just starting out is folks you work with or have interacted with in your space. These are folks that will understand your idea, know the market and will be able to buy into the concept. Also, you'll have some history with them and ideally a good working relationship (some work chemistry) and you'll have a sense of their expertise and they of yours.
I’ve been frustrated by the same issue in the past. What I learned over the course of sharing my idea with many people is that it needed to transition from abstraction into something visible. What I mean by that is there are multiple ways to make an idea intelligible to others so they grasp it beyond whatever words you use to articulate it over coffee. I recommend you do the following, if you haven’t already:
Create a slide-deck (5 to 10 slides) that provides a simple story in visual form for how customers are going to engage your product. You have to be your own creative advertiser and piece together whatever you can to spark that “oh, I see” reaction in potential co-founders.
If your idea is software related, creating simple wire-frames and UI goes a LONG, LONG way. You can do this using PowerPoint, Google Slides, or a prototyping service such as proto.io or prototyping on paper. Exhibiting the step-by-step UI process shows a potential co-founder the logic behind your thought process and also makes your idea a little more “real”.
Fill out various business-plan templates such as the Business Model Canvas, a Value Proposition Canvas, a SWOT analysis, etc. and show them to the people with whom you are interested in collaborating. Even if you can’t complete every field, doing what you can shows that you are filtering your idea through various strategies.
When you go through each of these processes, you will likely find that your idea can change a little here, a little there. That’s what visualization does, it gives your idea clarity, not just for others, but for yourself as well. In short, people need to SEE your idea, not just HEAR it.