I don't know the exact reasoning why
Java is designed that way, but at a guess, I'd say that it would violate encapsulation.
Assume for a moment that you
could include instance variables in an interface. The methods and members of an interface are public. If you implement the interface in a non-abstract class, your class's required instance variable will be public (since you can't reduce the accessibility). That is not so good, so you can't do it.
Hope this helps,
jdmaddison