Interface variables are static because Java interfaces cannot be instantiated in their own right; the value of the variable must be assigned in a static context in which no instance exists. The final modifier ensures the value assigned to the interface variable is a true constant that cannot be re-assigned by program code.
Since interface doesn’t have a direct object, the only way to access them is by using a class/interface and hence that is why if interface variable exists, it should be static otherwise it wont be accessible at all to outside world. Now since it is static, it can hold only one value and any classes that implements it can change it and hence it will be all mess.
Hence if at all there is an interface variable, it will be implicitly static, final and obviously public!!!