Maybe they're in desperate needs of money and they can only rely on what they can get from bounties. Some also want to grab the chance when the value pumped and never look back again (no ragrets. Lol). I think everyone have their own good reasons why they will sell.
The lack of understanding on how the crypto world works may be the 1st factor to be afraid of. Also the trust in the project. As we all know there are a lot of frauds, some may be good at first but will left all of a sudden lately. And how the mind works, negative comments will always linger more than positive.
Altcoins can be a better choice but you need the best decision to choose a good one, with a high potential project. Of course you also need to put an eye to their projects for you to know your next move.