Looking back, most people have decided that marker interfaces aren't an especially good idea, and so they're little used in new code.
Java 5 introduced
annotations which are, in effect, a way to introduce new keywords to be used the way marker interfaces were.
But to answer your question: adding a new keyword to a language is a drastic step that always breaks some existing code that just happens to have used that keyword as a variable name. It also complicates the language grammar and therefore the compiler and the language reference manual. In general, Java's designers wanted to avoid these issues.