[Organizers] Saying "no"

When you're handling booking for bands or callers, there are generally some proper you don't want to book. When the reason is straightforward it's not too hard to communicate ("you need to get more experience; play some smaller dances and get back to me" or "on even days our series only books local callers, ago the dates you suggested don't work, but what about DATE?") but sometimes the reason is "we don't think you're very good, and you've been doing enough dances that we don't think booking you for ours will help you improve"? I'd like to just say "no" without getting into reasons, but everything I write sounds either too short and curt or overshary. How do other bookers handle this?