Because of "The Deadly Diamond of Death".
Lets consider this situation:
You write class (name: Parent) in programming language where multiple inheritance is allowed. That class contains one instance variable ( int a;) and one method (void hello()). Then you create two classes (Kid1 and Kid2). Every of that classes extends Parent class. Then you override hello(); method in both Kid1 and Kid2 classes and you assign some value to a variable, also in both Kid1 and Kid2 classes. In last step you create 4th class (like Main) extending Kid1 and Kid2 classes (multiple inheritance). Now...question is, which of hello(); methods will Main class inherit and which value of a variable.
Since there is no multiple inheritance in Java, we have interfaces... abstract classes containing abstract methods. We can implement multiple interfaces, but under condition we have to override all methods which implemented interface contains.
