Multiple interfaces can be implemented, but only one class can be extended. A completely abstract class is a lot like an interface, except that an abstract class can contain variables.
It really depends on what you need. C++ allows you to extend as many classes you want, and it turns into a bit of a disaster. The nice thing about having only one superclass is that there's only ever one other set of implementations that you have to worry about (even if the parent has a parent, the parent's particular combination becomes your parent...)
Interfaces allow one object to play many roles, but they don't allow code reuse.
It's really to simplify thinking about inheritance. On the balance, I think they got it right.
