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.
Of course they can see it because bitcoin is already there and the documents of it are already laid out, they can just read it to know if what is bitcoin for and why it is built. The vision of btc is for the people to have a currency that is free from banks and government regulations.
If there are things that btc is not good at, that is the speed and the fee. They can create a coin that solves this issue but the problem is someone already built a coin like this so what are the other coin developers after at? it can be clear that they only want is to profit. Greedy coin devs didn't fail just because their coin died but they are successful because they already earn during the pre sale of their coin.