When it first started out you needed an invitation. Gmail had the same thing. Google does this fairly often to prevent still developing services from being crushed by an onslaught of signups.

G+ never gained traction because it was always the nerdier also-ran to Facebook. Everyone was already on Facebook and G+ didn't really offer anything compelling to make people want to switch. It had (has?) somewhat better segregation of your friends but keeping track of all of that is too much hassle for most people and doesn't really buy them much. Trying to decide if only your family would appreciate this witty cartoon or if you can also include your work colleagues is not something people want to do on every post.