I can sympathize with your frustration, my last project had two great voice actors bow out of a role (which I ended up voicing myself)... you can't really be too hard on volunteers though, they have every right to do so. We're all human, never forget that.
All I can say is don't always wait for people to come to you when casting auditions, go out there and find people too. I'm pretty sure there are people out there who don't go to auditions yet are talented actors... give them their chance to step up to the plate. You have very little to lose.