If I had to guess, I would assume it's similar to the Verge/Engadget situation. Splitting from a stodgy corporation that doesn't want to change or update anything.

Eventually they collectively realize that nothing is going to change and they could run a much better site without their parent corporation, and it goes from there.

Of course, this is complete speculation based on zero actual evidence. But it would hardly surprise me if the WSJ and AOL had a reluctance to be proactive in common.