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.
I think most of the new altcoins created just to earn on pump and dump strategy so no wonder they disappear afterwards. So the team you are talking about probably decides to give up midway because they already earned what they wanted.
Of course not all of them are like this. There are at least several established altcoins such as Dash, Dogecoin, Ethereum, Litecoin, Monero and some others.