[blhack]

...because we don't need to? Earth is already full of plant life. Mars isn't.

Sorry for the overly blunt answer, because I know this isn't the question you're asking, but IMHO the question you're asking is based on a ridiculous premise.

We are polluting the Earth. Yeah, this is really bad for the Earth, and for us, but it isn't going to kill all life here. The thing that a "backup planet" is here to protect against is something like being struck by a meteor, or a plague, or nuclear war. Something where everybody dies very quickly, not something where life changes radically.