In D&D that is known as the edition wars. Psychologically that is the same well-known effect as people being angry about somebody playing a different MMORPG than they are. Pen & paper games as well as MMORPGs consume so many hours, that they become akin to a lifestyle choice. And somebody choosing a different lifestyle than you are is perceived as a threat, as it calls into question whether your choice was the right one.
While one might have trouble overestimating the inferiority complexes and senses of entitlement on display in many online discussions, another take is that choosing a different MMO or edition is a threat because it diverts resources away from your choice and increases the threat that your game will fall below the critical mass necessary to maintain support for it.
The fewer people that play your game, the harder it is to find people to play with. It also means less developer time and effort being spent on your game, because there are less resources to support them. In MMOs, that can lead to games going offline, at which point you cannot play anymore. Pen and paper games do not have that risk, but to the extent that there is value in having a supporting gaming community, you need them to support your game.
And the closer someone is to your game, the more problematic it is that they are not playing your game because they could be. They face the exact same problem that you do, in terms of needing attention on a particular game, but they are making that exact same problem worse by going with you 90% of the way and then diverting attention at the last moment (and of course the fools see you as the one causing the problem). Maybe this will be the expansion that radically expands the playerbase, as WoW did for MMOs, but most games are competing for the same pool of players. The scarcer resources are, the uglier the fights over them tend to be.