I think it's probably more to do with the (perceived) intelligence of the audience, and how well they understand the game. Firstly rugby is more complex than soccer and it's very common for pundits and commentators to explain some of the more unusual laws of the game as they come up, or try to shed some light on a referee's interpretation. So there is already an understanding that talking about rules and complex areas is something that is done routinely in rugby.
Furthermore, though it is obviously not always the case, there is certainly a perception that the average rugby fan is a fair bit more intellectual than the average football fan, so perhaps intelligent discussion is more encouraged in the rugby studio. Plus, it doesn't take a lot to notice that the pundits are certainly more intelligent, most former rugby players having been considerably better educated than their footballing counterparts, so perhaps they are more inclined/able to participate in intelligent discussion themselves.
Personally I would absolutely love to see some of the radio 5 pundits (Waddle, Nevin, Marcotti for example) given a chance on TV, and the same goes for other journalists who appear on for example the Guardian's football weekly podcast (Rafael Honigstein, Philippe Auclair etc). These guys are scholars, students of the game, and they provide genuine insight that is sadly lacking from most television coverage. On the other hand, it might be inaccessible to the majority of football fans who watch the TV coverage, so perhaps it's a niche that is already sufficiently catered-for.