Because multiple inheritance causes the diamond problem. C# forces you to solve it by declaring one method as the "canonical" and the others as "explicit interface implementations" (only accessible if the variable/receiver is typed as that interface).
The diamond problem strictly speaking only has to be one when the common base class has constructor arguments. While a Java-style interface construct makes it easy to prevent, it also imposes much stronger restrictions than the above. It would have been possible to only impose the above restriction. Yes, there are failure cases with separate compilation, but the way Java dynamically loads classes, that would be similar to how when the JVM loads a class file that is supposed to be an interface, it discovers that instead it has been changed to a class.
