JDK 8 и знак подчеркивания (_)

Потому, что он зарезервирован для использования в будущем.

We are "reclaiming" the syntactic real estate of "_" from the space of identifiers for use in future language features. However, because there are existing programs that might use it, it is a warning for identifiers that occur in existing syntactic positions for 8, and an error for lambda formals (since there is no existing code with lambdas.)