There is no "why", only "how".
Whether or not there is life on Mars, or anywhere else for that matter, has to do with local conditions and blind chance. In nature, nothing HAS to happen, and nothing is forbidden, as long as the bookkeeping works out OK. There are no profound philosophical principles driving everything to a preordained and majestic conclusion. I suspect life CAN arise in the universe wherever conditions exist that will allow it. That does not mean it HAS to arise. Like I said, the distinction is subtle, and not everyone seems to want to get it.
There is no Force, Luke. The only reason we have a Grand Canyon is because water flows down hill.