The court basically said it was a separation of powers issue. The basic powers of the branches are:
The Legislative (Congress) creates laws
The Executive (President) actually puts those laws into action (they are “executed” by this aptly named branch)
The Judicial (courts) interpret legality of the actions of the Executive branch based on the wording of the laws passed by Congress, and the constitutionality of those laws (that is, if the law itself is even legal to be enforced)
The Chevron Deference doctrine was the courts saying “Congress occasionally writes laws vaguely and we don’t have expertise on every subject matter, so we are going to defer the decision-making of what exactly the law means to actual experts in the Executive branch.” Congress has written laws using this logic, intentionally granting power to the Executive branch that would otherwise reside with Congress (i.e. Congress says “how much of X particulate in the air is too much? We could write a specific law stating that 500 ppm is too much, but it’s a lot of work to do that for every particulate, and the science gets updated over time, so we’ll just tell the Executive to place ‘reasonable limits’ and call it a day.”)
Now the Court has said “That power you’ve ceded to the Executive branch? That should be ours because it’s our job to interpret what laws mean. We now decide how much of X particulate is too much, even when we mix it up with Y particulate.”
It’s a blatant power grab by the Court and a separation of powers issue. Congress SHOULD be able to remedy it by specifying that this decision-making power should reside with the Executive branch and the Judiciary won’t be able to say “no mine”. I mean, this Court WILL, but a legitimate Court wouldn’t.