Why can some operators like '::', '.', '?:' not be overloaded in C++?

:: works on namespaces and type names, you can't overload that because types aren't something you can return in C++.

?: could possibly be overloaded as long as you give up its short circuiting aspect, since you'd have to receive both branches as parameters. Not worth the drawback in my opinion.

. could definitely be overloaded, it has the same logic as -> (with slightly different binding, of course). The reason why it's not is anyone's guess however.