Because their tokens do not interest to buy them. When token is not much that interest and sales do not reach the sales target then the ICO will certainly fail.
Some coins failed after ICO for a lot of reasons, poor Marketing strategy, no support system, fundamentals are unstable, investors are already committed to other coins project, and a lot more other issue that may affect the level of interest of public investors. some say, if once coin survived the 18 months in the trading market, it will slowly get the movement of the market traders interest and possible investors. Not all Coins after ICO will be in the trading market in the future.