There are many reasons for this. You need to know what exactly happens inside each project. Sometimes the organizers are afraid that investors will not waste their Ethereum or Bitcoin on a falling market. Sometimes it comes from greed, when the team thinks to raise more money this way. But most often this extension ends with nothing.
The for most reason that they say is because of unfair market and now that they extend the ico they will have more chance to gain investors or investments? I doubt but i know that any extra extension will add some little more fund to the total amount collected.