Perhaps they're not sure if they'll stay interested in the character. If they lose interest and want to try something else that also ends up forcing them into a player faction, that faction would be (understandably) hesitant to accept the application. And there are no rules forcing factions to accept applications - which is a good thing, so long as there's no rules (save for obvious exceptions) forcing you into factions.
Edit: There's also another concern: some factions are slow to respond to applications.
