It's nowhere explicitly mentioned in the film as it obviously tries to steer away from eco-political issues, but it should be plain obvious that Blight is not a mysterious plague suddenly striking a planet that's otherwise in a perfect state but a plant illness that now exploded, in opposite to those millions of years previous to that, when Earth had a stable and healthy biosphere and that illness was controlled by nature's balance. Obvously, the balance toppled, now in favor o the illness.
No matter what habitat and how perfect the environment - as soon as you grow more than just a few plants in a lab, you will always have either parasites or microbes or spores liking a plant just as you do, sooner or later you will always have to help a plant by some means of assistance: a bit more light, a bit more or less water, more or less fertilizer, some antiherbal or antiparasitic remedy, whether chemical or biological. It's impossible to evacuate people from the planet on a larger scale without bringing some spores and microbes surprises with you. The difference is: in a clean habitat atmosphere on the cooper station, for example, if done right, then those remedies will work again like they once did on Earth. The plants on Earth were weakened first before Blight could kill them.