99% of candidates don't even get into the salary discussion phase. It's hard to hire because candidates think they know more than they really do. Some have 5 years experiences, many have 1 year experience five times.
So these are all people who have agreed to undertake your presumably time-consuming interview process without even a hint about financial compensation at the end of the tunnel?
If so, it seems extremely likely that you are pre-screening out a huge pool of the most talented and intelligent developers. It takes a special kind of brainwashing to agree to jump through those kinds of hoops for a prize that is kept secret until the end of the process.