Winning once proves who's the better team on the day. If they were to play twice then they'd need a 3rd match as the decider if both teams won a game each against each other. Then there's club commitments then Internationals in June and November. It's a lot of rugby in a year and the body will get overworked and tired.
Not enough time!
That means a ten week committment for the international players, along with their mid and end-of-year tours/tournaments. It's simply too much.
It would take the players away from their league teams for too long, and increase the risk of injury.
That's what they do.
The teams in the premiership would struggle for example leicester Tigers have 7 England players out so they wont be able to do very well
Tradition partly. As well, the season would go on for too long depriving clubs of their players.
