As stated in Roni's reply, it's for convenience of both staff and players. Some people do not take critique too well and we don't want to spit the soul into the wound by really providing the reason why the player got denied.
I see some people complaining, about us not cleanly explaining why a certain player got denied - on second thought - would that really be that good for the community? Honesty is a good thing, but there are situations where it's better to say shortly denied rather than putting some harsh words to already disappointing result for one.
If one really wants to know the reason behind his denial, he can send a PM to the one taking actions by all means for a private conversation. The staff is not obliged to tell you the reason and it's his own personal choice. Some may not feel comfortable to state the reason and some may not feel comfortable to listen to it. I think it's the best of the both worlds to not state the reason in certain spicy cases and if both sides agree rather tell them privately.
I know personally, if my app got denied, I'd much rather go over it privately rather than having my flaws pointed out in front of the entire community.