First, let me play around with your question a bit. I'm not trying to be snarky, but I AM trying to widen the view a bit:
Why do threads often turn so toxic when [housing] is mentioned?
Why do threads often turn so toxic when [parsing] is mentioned?
Why do threads often turn so toxic when [the Mentor system] is mentioned?
Please think on that at a later time.
Now then, it's no secret that PvP is arguably the least popular feature of this game. From the limited mindset of "this isn't a PvP game" or "PvP doesn't belong in a Final Fantasy game", to the near sense of pride people seem to have for openly hating PVP, in this game or any other. You have openly nasty comments like "I wish they'd stop wasting development resources on PvP," or "PvPers are so toxic, that's why they don't deserve chat!" fired off regularly, as if fact and not the opinions they are, and often celebrated all the same. You get Resident Sleeper emojis and angry "nobody cares! Get on to more important stuff!" comments during live letter broadcasts, and NOW, you have a mass influx of people into PvP who don't know what to do, don't care what to do, and expect if not demand they receive rewards proportional to those who do, or openly exploit it for said rewards. And couple that with SE's poor handling, perhaps-not-intended neglect, and the recent sweeping changes that dismissed and alienated what was otherwise a dedicated part of the playerbase in favor of a mass appeal to a more fairweather audience that it still failed to legitimately attract.
Simply put, at almost every given turn, PvPers have gotten little, told they deserve less, told what they enjoy is a "waste of development", shunned, ignored, ridiculed, entirely misunderstood, then those same people want whatever little PvPers can actually get because "it's not fair that they only get it". . . I won't for a moment justify anyone being nasty about it, but don't you think people might be just a little irate to be subject to that? Consider for a moment how it might feel if it was something you enjoy most in the game that garners such hate and disregard. How would you feel?