The rational reason is that global variables are usually a bad idea so there is no downside to leaving them out of the language.
Then there is a practical syntactic reason: everything in Java needs to be defined within the scope of a class according to the syntax. So there isn't really anywhere "global" that you could put a named variable without changing Java's scoping rules. Arguably, the closest thing that Java allows within the syntax would be a "public static" member of a class which can be used like a global variable in many ways.
Finally there is an often forgotten principle of simplicity that comes in many guises: "if in doubt leave it out". Or "Keep It Simple, Stupid". Or YAGNI. In language design, often less is better in terms of features. On balance, Java is remarkably well designed and has stood the test of time from this perspective.
