Why do you think good altcoins fail to keep going? It seems there are some that are good and people collectively have put in tens of thousands of hours supporting, but the team decides to give up midway.
Because what you consider as "good altcoins" are either scams or that there really is no need for them and they are all riding their own hype by taking advantage of the economic law of supply and demand, and the typical altcoiner's gullability and ignorance. The demand part is fake because, as mentioned it is derived from hype. The supply part is created out of think air just like in the case of ICO tokens and proof of stake coins.