It is most likely not the greed of people that leads to a periodic fall in cryptocurrencies that seemed promising for the development, but the herd feeling when they follow the crowd. In finance, this should not work. Here the situation largely depends on the advertising of the product and the rush demand created on the basis of it. Of course, it is difficult to resist mass psychosis. But you need to learn this and develop your attitude towards each cryptocurrency and the prospect of its growth.
Of course, there are many other factors involved as well. But this is probably the main one.