Microsoft has done a good job of improving Bing to the level where it is generally comparable with Google. However, Google still has a huge grip on the share of web searches, controlling 90% worldwide and 80% in the US. This huge lead could be attributed to many things, one being defaults. Google is the default search engine for a lot of different web browsers and devices. Now with Apple's contract with Google expiring soon, the default search provider for Safari could change.
Bing is currently powering Spotlight search in OSX Yosemite and iOS 8, but not in Safari. It will be curious to see if longtime rivals Apple and Microsoft see a common enemy in Google and will try to cut down on Google's reach. Although there is another search engine which has been in the news lately for being a newly chosen default, that is Yahoo. How Apple makes the decision to choose their default browser is not public knowledge but there are many factors for them to consider.
Apple has huge name recognition and is the most valuable brand, but that doesn't mean their products are the most popular. Apple has lost considerable market share to Google in smartphones and tablets over the last couple of years. On the laptops and desktops, the picture isn't any better; Apple has never been able to recover a significant amount of the market from Microsoft, and OSX is only on 6% of computers worldwide and 11% in the US. This means that whoever gains the bid for default won't have a serious impact on their overall market share.
All things considered it is a complex decision to make, since Apple has to consider what their users want, what the company needs to stay competitive, and what is the lowest bid. Choosing Bing to rule Safari's web searches may be the best choice for Apple because Bing is already the default on iOS and Spotlight. Ultimately this decision gives Apple some power to damage Google since the vast majority of Google's revenue is from web searches. Who will Apple choose in the end, we'll have to wait and see.