Also, YC can't accept every applicant, even if that application can accept the solution might change as time goes on. Maybe it's that YC just saw a pitch a few

Also, YC can't accept every applicant, even if that application can accept the solution might change as time goes on. Maybe it's that YC just saw a pitch a few hours/days before about a similar product with a better team/solution. Maybe it's that YC already invested in a few similar companies in the same space. Maybe it's the lack of a track record of the founders in building things. Maybe it's the black shirts. There are a lot of reasons besides being tied to a solution.