One of the main things that led to G+ waning was that it was frequently misrepresented by journalists as being nothing but a Facebook wannabee. Many people who came to G+ were also confused by the "broadcast" nature of G+ having expected the "friending" paradigm of FB.
There were numerous people who thought that anyone who added or followed them had access to private information. Many accounts failed to post any content which allowed for discovery.
Google was so focused on building the number of their accounts that they failed to communicate the main differences between G+ and FB for fear that it would reduce the number of migrations from FB.
However, there were also two things which Google itself did that were self-destructive. The first of these was featuring dead accounts in the Find People tool.
The second of these was introducing a ham-strung new interface. If the new interface had provided for the same features as the old interface, this would not have been a problem. But, it did not.
Many users became so disgusted with the new interface that they simply quit participating.