I believe developers saw the the vision of Bitcoin and try to explore from the vision by introducing and making their own coin but they lost focus because they only have one goal in mind which is making money, forgetting the most vital ingredients in developing an asset. They forget that they are certain steps and strategies to undergo before accomplishing a good assets or goal.
Some of the reasons why I think some developers fail is because:
1. Lack of funding both the starting aspect and the launching aspect (lack of money to continue)
2. Failure in out righting frauds
3. Failure to evolve especially when the coin initially start bomming
Most developers have only one vision which is making money that is why many coins doesn't grow and they failed because they didn't see the vision ahead.
I think why many altcoin projects have failed because they only sell coins and give some bonuses to each buyer through sales, but they don't offer a long term program that they will do in the future. because I think many investors are interested in projects that have long-term prospects by launching various programs and products. so many projects fail because they only build short term projects.
I think that the reason why they fail is very simple, those who create cryptocurrencies do not all do it with good intentions, the intentions that do it the most is to profit themselves through them, however when they do a good job and achieve good community, the coins can be maintained over time and become coins that only survive in Pump and Dump, the ones that have the most relevance are those that reach Binance and from there they can make those movements, this is very common and I think that for that is the reason under which there are so many crypto projects, it is a lottery to say which project is worth it.