That is a coven decision and some covens keep the reason private. Covens do that when they feel like they have too many members, going under construction, etc. Just wait if you want to send an application or you can message the leader(s) for any future reference regarding when the coven may accept members again or other things.
I don't believe there are any limits to how many members each coven may have, but I think that somepriest's and priestesses like to have a close-knitted group maybe so their coven is easier to maintain, or so it has a more 'family' like quality to it. I am not a priestess so I am only conjecturing.