Last Week Tonight has some pretty good investigative reporting, but there's also a significant difference between Oliver and Jon Stewart's Daily Show. Stewart confronted people directly with interviews, whether they were on his show or on his ideological opponent's, and his criticisms were widely discussed on talk shows the following few days after his points were made.
Beyond that, he is on HBO which one could argue is further into the "privileged liberal bubble" or however one would like to spin it. In general I don't think people pay attention to Oliver beyond fans of his work. I don't often see his stories being referenced via other news outlets.
Feels unfortunate that no one can really fill Stewart's shoes just yet.