Because we havent had a great track record of introducing (intentionally or not) new flora and fauna to new ecosystems on Earth. Australia is a good example. We are not technologically proficient enough to predict and factor all variables that will create a homeostasis. If we try now, it will probably create a runaway chain reaction, a Frankenstein, a bigger problem that future generations will need to solve. To donit correctly, we would need a create and test a Martian biodome on Earth. There's also the big question how and who decides what Mars should look like and how we as humanity plan to utilize it. This political question is becoming more pressing, as governments and private corporations are increasingly setting their sights on exploratory missions to the Moon to develop permanent bases and asteroids for natural resource extraction.