The thing about rivalries is that they're only as good as the teams are at the time. The Souffs vs Roosters rivalry was dead in the water when Souffs were rubbish and nobody has ever really cared about Cronulla vs St Merge because they've not both been good at the same time for any length of time. Cowboys vs Broncos only really got significant buy-in outside Cowboys fans when the 2 teams were genuinely competitive. Manly vs Melbourne has stayed strong but it's only about 15 years old and Melbourne have been at the top for most of that while Manly haven't dropped away for more than a year or 2 at a time. Also, rivalries come about because of history, they can't be synthesised. Nobody would have picked Manly and Melbourne to be rivals, it just came about through the course of history, it's what gives the players, the clubs and the fans something to actually care about, not just the other team is close to us.