She dated Peter Gabriel sometime in the 80's I think it was. She was dating Steve Porcaro from Toto at the time, Steve and her met with Peter and his band. Rosanna and Peter (who was in the process of splitting with his wife Jill) took a shine to each other.

Well it's Hollywood, seems most marriages in that town end up in divorce.
Y'know, I could eat a peach for hours

I have a theory (& it's not judgmental): Maybe marriage isn't for some people.

I would say leave it alone - not everyone is cut out to stay in long term relationships for whatever reason. In my opinion it is really nobody's business whether or not she has maintained any long term relationships nor who left who.
However, in an attempt to at least respond to the question, she and her sister Patricia were being interviewed some time after brother David Arquette's most recent public episode involving substance abuse and they intimated they all had a very troubled home environment when they were growing up and hinted at physical and emotional abuse which may have contributed to David's substance abuse issues. She also is quoted as saying she is very insecure. Either of those issues could cause someone to have unstable relationships with others but who are we to say?
Maybe she just prefers to not have long term relationships. Either way I just hope she is happy with her life and herself. I personally am one of those people who is quite happy without a long term partner for reasons of my own so I don't question any one else's relationships, their problems with those relationships or lack of long term relationships that appear to be outwardly stable. We are all very different people.