Mars declared independence, and Earth didn't like that. Until that point we'd been a colony, and all our resources, everythin' we mine and made and gained was destined for Earth. So, they didn't exactly take kindly to us pulling out of that. For a long time, there was a very real threat that we'd all wake up to nothing but nuclear bombs and ash.

Terraforming didn't matter as much, when we didn't have the power to protect it. Now we do, but that means the blue sky and the green grass has to wait another fifty or a hundred years.