RU internationals have two massive things going for them:
1) Union internationals are seen as EVENTS, not just sporting contests.
2) The rules of union enable a team with a strong pack and no passing game to compete with a team they are light-years behind in terms of running and passing.
If you compare to RL, far too many RL fans are seemingly purists to the nth degree, or club first, second, third and fourth, and happy to boycott games etc on the grounds of player X being selected because not from their side or because despite following the same rules as other sports, is in some ways not 'English'.
Then you must add the fact that the parts of RU that enable competition between sides with outrageously mismatched back divisions - i.e. contested possession, ability to keep a game tight and value of penalties - is simply not possible in RL. I'd argue England RL is probably not dissimilar in ability differential as England RU when compared to say Australia. But whereas England RU can stuff the ball up their jumper and play for teritory and possession and penalties, England RL simply can't do the same, because Australia in RL will get the ball at least 50% of the time.