> The excitement was gone.

Most people didn't hear about it. You can ask people on the street. They sure know facebook. Excitement is not a good way to motivate people to move platforms, in general (despite the niche cases like Fyre Festival which are relatively small). Maybe they should have gotten some celebrity support? Otherwise the excitement wasn't mainstream. G+ needed mainstream users.

> But people did want to join Google+, but they couldn't

That doesnt explain the lack of participation a year later. Nobody wanted G+ because they didn't need it. Even if it gave some benefit (which it couldn't) it didn't join with their existing Social Media data making a higher barrier.