Adverts.

Spatial adverts are a big thing now, with a big future. I would speculate that beyond the cost of licensing the maps or the contractual stuff is that Apple did not want to deliver to Google more advertisers that would further entrench Google Maps as the dominant player.

Apple need to be able to build their own POI database and knowledge of where all advertiser businesses are in order to get that revenue long-term.

Thus, Apple need control of the maps to control the method that advertisers get onto the map.

I think it's just strategic rather than contractual or short-term financial.