This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
Well, in short, because in Java, abstract method implementations are looked up at runtime by checking the exact type of the object the method is called on. Since static methods are called without reference to an object -- i.e., you can call them as ClassName.methodName(), with no object involved -- there would be no way to figure out which implementation you mean. Of course you could work out a system for doing this, but Java's creators wanted the language to stay simple, with relatively few rules for understanding its behavior.