This is one of those questions where the answer presupposes the question. If this planet hadn't been so hospitable to the development of carbon, oxygen and water reliant life, we wouldn't be here to ask the question in the first place. Hence there are no human beings on Mercury saying "Why is this planet so inhospitable to our kind of lifeform?".

The question it does raise however is just how perilous and unique are the conditions that exist on earth, and could life develop under other circumstances. My own personal view is that there is undoubtedly life elsewhere in our universe: after all, even if the odds of life developing here are a million to one against, or even a billion to one against, the fact that there are more stars in the universe than grains of sand on earth means that single-cellular organisms or even basic protein chains will have developed somewhere; whether any of them ever evolved beyond that point is the real question.