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.