How to get feedback from brand owners and influences? How to intreprete good feedback but no real interest?

I have developed a web & mobile app for a specific fitness niche and I would like to show it to niche insiders such as brand owners and influences.

I had contact with the owner of a large brand in this sector, showed him my app and proposed a collaboration. He told me he really likes the idea but in the end he declined my proposal because he and his team were too occupied with other topics. Honestly this was really disappointing to me because at first it looked like it would work out and I was very hopeful.

I have invested a lot of time into this project and do not want to give it up, even though at this point I'm unsure as if I should continue.

I have already tried cold emailing but I almost never hear back from them. I'm not sure if that is the case because my app/idea is bad or if they just don't really bother looking into it. When I rarely hear back I have never gotten any negative feedback. It's always something like "I love the idea and your implementation but currently we are not interested because ...".

Should I accept that this idea is not good enough or should I keep trying? Is there any better way to get in contact with insiders of this niche?