Each coin has it's own reasons.

Many have unethical launches. This includes "premining" or mining a coin for a bit, and then starting to spend your advertising budget promoting it, resulting in the founder having a large amount of the coins without doing much computational work.

Frequently they have alot of code similar to Bitcoin, generally with one or two changes that the creator argues makes it superior. It doesn't matter how close the code is to Bitcoin, if it's not Bitcoin, it's missing a couple of things; Bitcoin's network effects, and it's infrastructure.

It is said that a competing network has to be 10x as good to supplant a network. In addition, Bitcoin has tens of thousands of nodes that have copies of the blockchain and verify the transactions themselves. These forks would not have any of these nodes and start at 0. Changes, that the creator claims to be improvements, most likely will have unintended consequences. For example, increasing the block size (like BCH, or BSV) or decreasing block times (LTC) both will increase the total blockchain size, increasing the hardware requirements of running a node, reducing decentralization.

Bitcoin forks that keep the same mining hashing algorithm as BTC can be vulnerable to 51% attacks. This is because the amount of hashrate on BTC's network is so much higher than, say, BSV's that a very small fraction of BTC's miners can switch to mining BSV, attacking their network.

Other coins, don't have competitive enough mining industries to produce Application-Specific-Integrated-Circuits (ASICs). Again, a large amount of GPUs, which were mining other coins can switch over and attack their network.

Monero, which has unique mining decentrazation due to its ASIC resistance (I hear GPU's aren't worth it either) and privacy some Bitcoiners may envy, runs into a different problem. It's privacy makes it difficult to audit. In fact, you cannot. Based on certain cryptography, the correct amount of XMR exist, but one cannot be sure. It is perhaps for this reason that XMR reached It's peak against BTC during the '17/'18 Bull market and hasn't returned to this height.

Z-cash is a little easier to audit due to opt-in privacy. However this suffers from factors of anonymity set size, among other things. Part of the mining reward automatically went to the Electric Coin Company and part went to fund Z-cash development. Eventually, the ECC stopped recieving funds and the dev fund became non-profit, but this development tax still exists. And Bitcoiners frequently call tax theft.

Check out the "decentralization trilema" changing an attribute of Bitcoin always has a (very predictable) outcome.

Value converges on the best money. Tell me a non proof-of-stake alt-coin that is a better money than bitcoin.

Bitcoin: Winner Takes Most or Winner Takes All? https://bitcoinwords.github.io/bitcoin-winner-takes-most-or-winner-takes-all

Bitcoin Obsoletes All Other Money https://bitcoinwords.github.io/bitcoin-obsoletes-all-other-money

Let's say you are an auto mechanic. Would you not provide advice to your sister before she buys some car you know will bring her trouble?

Let's use Litecoin as an example. Litecoin is not protected by hashrate that consumes 15 GW of power, like bitcoin does. Could a state actor do a 51% attack against bitcoin? Very unlikely due to the enormous effort it would require but it is possible if they were so inclined. Could a state actor do a 51% attack against LItecoin? Still prettty unlikely, but mostly because there's little incentive to do so .. not because the effort is beyond that of a single nation state.

And litecoin has the vast majority of the hashrate for its hardware class (Scrypt ASICs). Now ask the same for the second most popular Scrypt-based altcoin. Or third-most, etc. Being proof-of-work does not just magically give protection.

So one reason for being vocally against other proof-of-work coins is to protect innocents who don't understand the differences between those coins and bitcoin.

Or take Monero. It wasn't that long ago there was a hard fork requiring an update by each node for it to continue working. That's a level of centralization, to begin with. You might feel the trade-off is justified, but bitcoin doesn't really force users to make such decisions.

Another reason is simply, ... all altcoins and tokens see bitcoin a a competitor. And vice-versa. Bitcoin would likely be much further along if it weren't fighting shitcoiners all day every day. Being "less shitty" than other piles of shit does not make that plate more palatable.

Few Examples: Litecoin is literally a clone of BTC with tweaked parameters. However it does have its use as sort of a testing ground for BTC (segwit for example). Plus it still has a figurehead in Charlie

When Grin was released, miners were ready and so it didn't really become a fair launch as all the heavy duty GPU people gobbled up the hashrate.

ETH: 70% premine.

As bitcoin was released during a time where there was no fanfare it had the time to establish and decentralize itself which no other coin can have. You could call it the "immaculate conception" of a blockchain that no other will ever be able to repeat.

This is just a few examples. It's almost as if every PoW coin that isn't bitcoin has its own reason for being rejected by maxis.

Each for different reasons. Many are forks of bitcoin that messed with the code for one reason or another, and never improving on the original.

Exhibit #34514:

https://twitter.com/BitcoinNewsCom/status/1621015621629624322 [Nitter]

And that one should have been obvious. Lite is not even spelled the same as the Light in (Lightning). Now consider the poor schmuck that bought Bitcoin Cash thinking it was bitcoin.

Bitcoin is king. There is no second best.