It was for stealth. This does not absolve them of their legal and ethical responsibility to give credit where it's due, but it explains their actions above "they're dicks".
April 2010 is around when they would have started working on the maps back-end. Attribution would have meant dev releases would have indicated Apple using non-Google Maps data. They probably wanted to avoid that.
I noted in my original answer that "this does not absolve them of...responsibility".
Some of the comments were implying that Apple didn't give credit because they have a culture that is insolent. I was just trying to give context to the decision.
Sorry, reading and comprehension never was my strong suit. I just can't help but feel that by 'explaining' this behavior it is in some way condoning it. I see now that was not your intent.
It appears they've had these maps in their desktop products since about April 2010, so this explanation doesn't make any sense. (And if it did, then a simple switch to turn on attribution at launch time would have worked too).
April 2010 is around when they would have started working on the maps back-end. Attribution would have meant dev releases would have indicated Apple using non-Google Maps data. They probably wanted to avoid that.