You say "they couldn't collaborate" I say "Google really wanted to compete".
Remember that it was Google who decided to perform a fork because they had a different approach than Apple for how "multi-process" should be implemented. Google had an implementation that they didn't want to integrate into the Webkit branch - hence the Blink fork.[1]
Also Apple would have been happy if they could use the Google's implementation instead of using their own - they said that repeatedly [2]
You can also find more details of the story here. It looks more and more obvious that the decision to create a WebKit fork was more political than technical : https://news.ycombinator.com/item?id=5490242
[1] https://www.theguardian.com/technology/2013/apr/05/blink-goo...