Some products are either poorly developed or the team is not serious and the product is unable to make the project keep token values up.
I think that mainly the problems are different. When the token gets on the exchanges, people who earned it for easy tasks are starting to sell it for at least some money. Then ICO investors look at their investment going down and contribute to panic selling themselves. Not to mention that when the whole market goes down, these tokens go down along with it. Another problem might be that there are truly some issues with the token. It's mining process is going slower than expected, for instance, is what I believe to be the reason behind electroneum's failure.