The reason lies in the design-principle, that java does not allow multiple inheritance. The problem with multiple inheritance can be illustrated by the following example:
public class A {
public method x() {...}
}
public class B {
public method x() {...}
}
public class C extends A, B { ... }
Now what happens if you call C.x()? Will be A.x() or B.x() executed? Every language with multiple inheritance has to solve this problem.
Interfaces allow in Java some sort of restricted multiple inheritance. To avoid the problem above, they are not allowed to have methods. If we look at the same problem with interfaces and static methods:
public interface A {
public static method x() {...}
}
public interface B {
public static method x() {...}
}
public class C implements A, B { ... }
Same problem here, what happen if you call C.x()?