The second reason, again this is only my opinion, is the old trade-off between simplicity and completeness. Object Sequence Diagrams, Object Collaboration Diagrams and State Transition Diagrams in particular. In either case the initial statements about what must be true for inheritance to be valid applies - strictly. For example abstracting out the common behaviour between two classes where it is merely coincidence that they share superficially similar behaviour would be wrong. And I make no excuse for repeating this material after only a couple of paragraphs, it is important.
Inheritance is about expressing two things about a relationship between two classes.
Now this is not debatable. This is absolutely the defining nature of inheritance. This is a consequence of inheritance - not a defining feature. Single and Multiple Inheritance Single inheritance is very simple. A class inherits from exactly one other class.