First, let me correct this notion that you have, not all long distance relationship fail. Some fail because of lack of communication and intimacy. Most long distance relationship do not have much communication which is a must to keep the relationship going.
Lack of intimacy also lead to the end of many long distance relationship, many people like to have physical presence of their partner in a relationship, without this many may find solace in the arms of another which may eventually lead to cheating and eventually lead to an unfortunate end of the relationship.
Long distance relationships do not always fail. However, they are hard to maintain because of the lack of intimacy. There is not much you can tell your partner on the phone. It cannot replace the intimacy and physical presence of your partner.
In addition, many people like to have real live humans to feel and touch to put life into the relation. For them, going away and not meeting for months makes the relation lose its charm. Moreover, the romantic needs of each partner are not met. Time difference can also make you unavailable for each other, which complicates issues and breaks these relations.