Apple gets paid tens of billions of dollars per year by Google, which is over a third of the revenue Google brings in from Safari. They have an absolutely gigantic financial incentive to keep using Google. Even if they were to acquire Bing for free, they would still have to get its profits to over a third of Google’s Safari-attributed revenue just to break even on the switch.
If Apple were to acquire Bing, it would be about gaining control at a financial cost. Bing being as good as Google as a search engine doesn’t really come into it.
Apple probably agreed that acquiring Bing was worthwhile, as the discussions did happen, but they haven’t agreed that it was worth more than the amount Google pays to them to stay the default search engine.
If Apple were to acquire Bing, it would be about gaining control at a financial cost. Bing being as good as Google as a search engine doesn’t really come into it.