Gavin Kilduff, a professor at NYU’s Stern School of Business, argues that a true rivalry has three main components: it comes from competitors who are similar, evenly matched, and face off repeatedly. According to Kilduff, where those elements of a true rivalry are present, a sporting event rises above the level of wins or losses or standings into a psychological event, the weight of which goes beyond the stakes of the game at hand.
For years, the Mariners have lacked any true rival, largely through their own ineptitude. I’m assuming Kilduff is using “similar” to mean “teams chasing a pennant”; the Mariners have been similar to other bottom-dwellers in their division, but not in a way that spurs any true rivalry.