Some people had their google+ accounts suspended for reasons such as "google is not sure that you are using your real name" or an unspecified terms of service violation and got their gmail accounts locked as well.
For many people, loosing access to their gmail is really bad. Suddenly it's a huge hassle to get their car insurance policy pdf or email their landlord.
For many people, it was not worth the risk of loosing access to their main email address.
Advertising is all they really do. Their software or other products are ancillary and supportive to their advertising objectives --- and obviously subject to being discontinued on a whim.
Google+ was a thing for many people, and it was beginning to gain some serious traction. A shame, really.-