It’s the same reason many iPhone apps implement their own dialogues to ask about allowing notifications. If the user chooses ‘Deny’ in the system-provided one, the app can never ask again and the only way to turn notifications on later is to have the user go digging around in the Settings app, which few people will bother to do.

I take great pleasure in choosing ‘Allow’ in those custom dialogues and then ‘Deny’ when the native one pops up immediately afterwards.