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.