They're probably over-worked and not had the time or energy. I know a lady who runs my local cat rescue quite often gets offers of help, she says its really time consuming as the helper needs to be more thoroughly vetted than adopters, then when they realise the realities of fostering frequently change their minds and leave the rescue with cats they have no room for. Not suggesting for one moment anyone here would be so fickle or naive but this has been her experience.
If you are really serious, a personal visit is likely to be the way to get noticed. Then they can interview you without the extra bother of setting up appointments etc. If this doesn't work, contact your local small independent rescue and see if they can use you?