They say this is due to low usage and challenges with maintenance. Read the letter below:
In December 2018, we announced our decision to shut down Google+ for consumers in April 2019 due to low usage and challenges involved in maintaining a successful product that meets consumers’ expectations. We want to thank you for being part of Google+ and provide next steps, including how to download your photos and other content.

On April 2nd, your Google+ account and any Google+ pages you created will be shut down and we will begin deleting content from consumer Google+ accounts. Photos and videos from Google+ in your Album Archive and your Google+ pages will also be deleted. You can download and save your content, just make sure to do so before April. Note that photos and videos backed up in Google Photos will not be deleted.

The process of deleting content from consumer Google+ accounts, Google+ Pages, and Album Archive will take a few months, and content may remain through this time. For example, users may still see parts of their Google+ account via activity log and some consumer Google+ content may remain visible to G Suite users until consumer Google+ is deleted.

https://www.reporterswall.com/google-shutting-google/