The article literally explains the why of it.

It's a stupid why. Currently they get $8 billion for nothing. Purchase DDG (and set default) and they lose $8 billion, users complain about worse search results (just like maps) and Google just puts a banner on their homepage saying here's how to change search engine and gets the search traffic for free.

Just because apple makes plenty of money doesn't mean they should buy every random startup.

It doesn't really explain why buying a product that's primarily backed by Bings search index (at least for western customers) is better than making a deal with Microsoft directly for Bing access.