Individuals of course vary, and the vast majority of people involved in cryptocurrency are able to rationally assess each project on its individual merits. Many people's investment portfolio & ideological alignment also changes within cryptocurrency over time as the market changes & their own understanding improves.
That aside, there is a recurrent theme of BTC Bitcoiners having specific animosity towards BCH / BCH Bitcoiners - calling it "bcash", pointing to /r/btc as disingenuous behaviour by BCH supporters & so on. BTC advocates proudly proclaim the free market until they are being outcompeted on it, as is increasingly the case with BCH. Their aversion & hatred is a multifaceted issue, which may contain any/all of the following elements (many are interrelated) on an individual-by-individual basis:
- History: The largest, longest-running & most controversial conflict in cryptocurrency's existence is the Blocksize War between small-block & big-blockers over the original Bitcoin. Some individuals may have been personally involved, although as time goes on many people who were not involved at the time have developed strong feelings on the subject because of later entanglement as with any historical geopolitical/social conflict.
- Censorship / propaganda: BTC is affected worse than any other coin by censorship and propaganda spread among its user base. Much of the lies & cope is directly related to BCH, because of the dispute over the real Bitcoin.
- Personal animosity: Related to the points of history, censorship & propaganda above - many BTC coiners may have a personal hatred of prominent BCH advocates (such as Roger Ver or Jihan Wu), even for people they have never met or interacted with and know very little about. A far smaller group may have individual issues or altercations or personal dealings with people associated with BCH in their private lives which affects their judgement. Of course, Bitcoin Cash is decentralised so those personal beefs are generally irrelevant to the project as a whole, but not everyone is mature enough to separate their emotions in this way.
- "Affinity scamming": Due to censorship/propaganda, some BTC adopters believe the BCH branding is an attempt to con investors or leech legitimacy from BTC. Ironically, the truth is the opposite, BTC has benefited heavily from the Bitcoin brand despite not being the real Bitcoin. The BCH community defiantly refuses to rebrand for this same reason.
- Toxic maximalism: BTC has developed a maladaptive form of "toxic maximalism", which includes generic hatred of non-BTC projects & particularly BCH (the bigger the threat, the more hatred). See What is Bitcoin maximalism?.
- Cryptocurrency tribalism: All of cryptocurrency is very tribal, and this dynamic plays a role in BCH/BTC relations too.
- Hashrate competition: Unlike almost any other coin, BCH shares the SHA256 mining algorith with BTC & remains a credible / growing threat to earn back its hashrate & thus plummet BTC security / value. See FAQ on low hashrate.
- Financial investments: BTC supporters of course have their own net worth (and often reputations / social credibility) invested in their project. BCH is a direct & credible threat to that, both from a financial & social perspective - so BTC supporters will attack BCH in the interests of protecting that social/financial capital. If/when they convert to BCH, shifting their finances partially or fully to match, this incentive then disappears or reverses.
- Cognitive dissonance / Sunk cost fallacy: Many BTC supporters, despite beginning to see the many problems in BTC, are affected by psychological phenomenon such as the sunk cost fallacy or cognitive dissonance in attempting to reconcile their past/present beliefs, actions or financial investments with the failing BTC project & continued progress of BCH.
- Groupthink / ingroup-outgroup cult psychology: The most devout BTC believers tend to have social networks full of other BTC believers, so a hatred of BCH persists through self-reinforcing groupthink.
- Fear of a Flippening: Related to history, hashrate competition, cognitive dissonance & "affinity scamming", BCH remains a viable & serious threat to "flip" BTC as the #1 cryptocurrency - causing calamitous losses for BTC investors. This makes BCH a specific target for attack in a way that most other (flawed, non-threatening) cryptocurrency communities do not justify.
- Poor price performance: Some pre-split investors may feel disappointed by their BCH holdings historical price performance and thus salty they didn't make different investing decisions. Post-split BTC coiners may simply use price charts as a convenient point of mockery. The BCH price has performed poorly, but there's clear reasons why & a concerted effort for things to be different in future.