Because it wasn't the best asset. You don't. Sometimes this is due to low expectations of investors, sometimes with a large number of bounty hunters.
It can be pump and dump or only dump. Bounty hunters and investors during pre-ico dump their tokens, bounty hunters because they will earn whenever they dump and pre-ico investors usually get bonus token so it is win-win for them.
Tokens can be pumped, and when dump starts everyone is in panic and selling tokens no matter what.
It's a win-win if the price of coin touch the ico price but if that will be even lower than the ico price and they were actually getting a lot of disadvantages. That depends on how much order that will be putting by the speculators to support the buy order of that coin. I guess if that will have a very strong support and then the dump will not happen.