It was becoming a game of technological whack-a-mole on Google's back end to manage account information across apps. For example, was a user logged into Gmail also logged into YouTube? Were they logged in as the same person? How do resources get unified across different apps, since that's behavior users assume should work? What applications had authority to act on a user's behalf, in what contexts, And can we provide a better way to support that functionality than requiring a user to give their whole password to a third-party system? And when an account had to be banned for being abusive, what precisely got banned? previous to account unification, it was a shotgun depending on who did the banning and in what context.
True names was, in my opinion, a misstep. The account unification goal was a great idea.