Because of interfaces they can have multiple different common ancestors.
One could add a requirement that it only auto-converts if the ancestor is unambiguous. But then adding additional interfaces a class implements suddenly becomes a breaking change. And that might not be desirable.
For example suppose you make these types implement ISerializeable. This shouldn't change the behavior of your code, but if you supported that casting to common interface it would.
edit: Thought a bit more about it and noticed that this function already has exactly the same problem:
T MyFunc(T left,T right)
And this code doesn't compile:
ICollection r=MyFunc(new List() , new LinkedList());
because it can't decide which type to use as the type-parameter T. So the behavior of the ?: operator is consistent with overload resolution.
