The same reason why it's a bit of a red flag if someone has no long-time friends or has no job references. Because it hints at relationships going in such a way that the people don't even so much as want to talk ever again after, and the common factor is the person.
Why?
> I'm surprised you dated enough that there's a "breakup" if you don't enjoy each other enough to have something to salvage.
Some relationships are mistakes; not all of us are prescient enough to know how they will work out. Other times you simply grow apart.