List<?> is generic (wildcard), which means that it accepts from String to anything, that's why

List<?> objList = strList; //valid construction

while List<String> is not a subtype of List<Object>(But String[] is in fact a subtype of Object[], that's one of the reasons why generics and arrays don't mix well). For Object

List<? extends Object> objList = strList; //valid construction

because String extends Object, here means anything extends to Object is accepted.

First explanation:

If you define a List of Objects, than you state, that this List can contain anything you want. (This is some additional information you provide.)

If you define a List of Strings, than this is NOT a List of anything. It is a List of Strings.

A List of Strings is not the same as a List of Objects.

Technical explanation:

Let's say you have a class like this:

class Machine<T> {
    void process(T input) {
        ...
    }
}

If you then create an instance: new Machine<String>(), than the compiler knows, that your new object is ment to process Strings. It will fail if you want to make it process a Number, an Array or anything else.

If you create a more general machine, but calling new Machine<Object>(), than the compiler knows, that Strings, Numbers, Arrays and any kind of stuff can be processed by that machine.

The compiler will not allow you to cast the specific String-Machine (new Machine<String>()) to a general Object-Machine (Machine<Object>), because this would allow you to let it process Numbers, Arrays, etc. for what it was not meant for.