But Earth has different conditions than outer space. So to assume that Earth is typical compared to all other parts of the Universe is just dead wrong. Here, we have and had a huge concentration of organic and inorganic matter, liquid water and the insulating atmosphere to maintain it, strong protection from cosmic rays, a relatively stable range of temperatures, and a mix of chemicals washed all around by ocean currents and interacting in varied temperatures and conditions. Mars had some of these factors, to a much lesser degree, for a lot less time. Asteroids? Comets? Both batting zero for zero.

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"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
Illuminati's Razor-The most complicatedly evil answer is usually the most correct answer. - Fazor
"Every book is a children's book if the kid can read." - Mitch Hedberg
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort