Some events are so rare or difficult, that the first time it happens is also the last time. Conditions have changed since the first time, and even if it should happen, it will quickly be killed by life that has had billions of years to increase its efficiency. This is especially true when life needs a chain of several rare steps. For example, RNA needs to become self replicating. Then RNA needs to learn to make RNA that makes protein. Next, RNA needs to make the bilipid layer. Finally, DNA needs to take over some functions that RNA did. Each of these steps could have taken untold millions of years to accomplish. One argument that it only happened once is that all life is left handed.