Reasons? Many but basically that the ecological prorities of Lula's government are heading towards none at all. Two huge hydroelectric dams, a nuclear plant, authorization for genetically modified crops and the centralization of Amazonian matters in a different ministry basically meant she was being sidelined by the right-wing cattle-rancher and blindly developist anti-ecological forces.
It's, as I see it, sad news for the planet and sad news for Brazil as well. Once again it becomes evident that right-wing policies can be carried on by nominally leftist authorities. They had a name for that: violin governments, because they are held with the left hand but played with the right one.