Google+ was dying slowly to be sure, but the reason it was killed off is because they tried to cover up a data leak affecting over 500,000 users. [0] They've definitely killed off/abandoned their fair share of flagship products without warning (Hangouts, anyone?), but Google+ was shut down to minimize media controversy.

[0]: https://arstechnica.com/tech-policy/2018/10/google-exposed-n...