Why ABC's are more powerful 'interfaces' than interfaces

Question

A simple answer as to what the value of Abstract Base Classes are is that
(1) They act like Interfaces, allowing for polymorphic abstraction of multiple implementations
(2) They allow for default implementations, but insist on derivation
For this question, please set aside any value that default implementations provide, and concentrate only on the 'interface'-like qualities of ABCs.
Why did sun decide to make ABCs more powerful as interfaces, than interfaces themselves.
Or... asked another way, in the following code snippet
public void doCoolStuff( ObjectOutputStream os, Fooable foo )
throws CloneNotSupportedException, IOException
    Fooable foo2 = (Fooable)foo.clone();
    foo2.changeMyselfSomehow();
    os.writeObject(foo2);
}If I said the Fooable 'thingy' has absolutely no implementation attached to it whatsoever, then, what kind of thingy is Fooable?

Answer 1

Functionally: A class can only extend one class,even
if the parent is an ABC, but it can inherit multiple
interfaces. public class Foo extends ABC1,ABC2
{} // won't compile
Yes that's the common logic. And yet Fooable
absolutely can not be an interface, but can be
an ABC.Interfaces can do X that ABCs can't do. ABCs can do Y that interfaces can't do. Which is more powerful?
This box is longer, but that box is wider. Which is bigger?