Instagram only implemented "close friends" after years of a solid user base to research the usefulness of that feature. It's not that they couldn't think of it; ideas are cheap. A lot of it has to do with the stories feature, an innovation stolen from Snapchat that G+ predates. And G+ Circles was way more complicated.

Circles feature was also marketed the wrong way. The G+ ad showed a lady bad-mouthing her boss in a circle that doesn't include him. Who's gonna put that much faith in the separation? The entire point of a finsta is that it's not a supported feature, so you can believe that it's totally separate and also has a different name. You'd use the finsta to bad-mouth your boss or perhaps even less wholesome things.