In a lot of ways it’s like the much maligned hipster-ism: “I liked it before it was cool”. This is the case both in it’s masturbatory and superior dismissiveness, as well as it’s commitment to maintaining the authenticity of the fan group – criticizing new arrivals for their ignorance of the true meaning of the game, music, event etc and their mucking up the hallowed space.
Perhaps the key distinction to be made in this comparison is that the approach to music usually involves a rejection of the artist, and a labeling of them as “sell-outs” for their progression beyond what is viewed as their authentic form. When it comes to sports, the fan remains a fan – and is probably more excited about their team’s success than new fans – but they find it necessary to criticize the failure of new converts to undergo the same self-flagellation in defeat they deem a rite of passage for the “real” fans.
There’s also a certain pseudo-nationalism or nativism to the in-group out-group labeling – perhaps with Red Sox nation the “pseudo” should be dropped. Newcomers are crowding out people who have long been participating and proud citizens, and one cannot be a true Patriot (see what I did there?) without conforming to “my country right or wrong”, or sticking by a team through epic failure as well as glory. This can have the constructive result in sports of creating and maintaining large and loyal fanbases, but it also can have the destructive results that are codependent with nationalistic mindsets.