Why is there only one common ancestor? Why is there no evidence of other "common ancestors" which were not as successful? Was there no competition to be the "first" common ancestor?
There are certain basic building blocks (eg DNA/RNA and the genetic code) to all known life on Earth which implies a specific way in which life came to be - 'the common ancestor'. Until the mechanisms of how that initial life came to be are known for sure, or at least to a high degree of probability, then the 'why' and related questions are unanswerable really. One of the reasons finding evidence of life elsewhere within the solar system (and beyond) would be so important is that it would help to begin to identify the causal factors more precisely. But so far as we know currently, life exists on Earth and it's all based upon the same basic building blocks and has a common origin. If there were other forms of life on Earth then they are obviously no longer with us and would seem to also be beyond our ability to detect. I don't think the mystery so much lies with 'alien' life on Earth, life here is adapted for Earth, but in how (if) life develops elsewhere and in different conditions.