List<? super String> can take a String or a superclass of String as a type argument. List<String> can have only a String type argument. The compiler cannot be sure that you would always pass in a List<String> since List<? super String> also allows a List<Object>. Hence, disallowed.
SCJP 5.0 | SCWCD 1.4 <br /> <br />If you don't know where you are going, any road will take you there!