Why is environmental policy so difficult? Granted, all democratic actions are difficult; it is the nature of democracy to sort through the opinions of the vast majority and, through the process of compromise and allowance, come to a mutually suitable action.
Environmental policy is particularly difficult because it is so far reaching and encompasses so many different conflicting worldviews. Actions that are aimed to protect the environment require modifications and restrictions to people's every day lives. Regulations can often be costly and invasive. Though these regulations may have benefits, those are often unclear or will only be seen some time in the future.
Likewise, the need for policies to protect the environment is not entirely clear to everyone. It is by far easier to see how policies restrict oneself from something than it is to see how one's actions is helping destroy the planet. In environmental terms, this is known as the "tragedy of the commons," an idea first coined by Garrent Hardin, or by the economic term "externalities."
Both of these concepts point to the same thing: when we are living, producing, and "developing" our world, we cause unintended consequences that are difficult to see. Things that are held in common, such as the sky, rivers, oceans, wildlife, or the sun's energy, are used without consideration by all. They are viewed as limitless resources and everyone tries to use these "free" resources to their advantage. In the end, the commons are depreciated and we all are worse off for it.
Externalities are the reason why we need government action but it is also why the action is so difficult to approve. Cutting people off from those "commons" by limiting what they can consume, waste, or effect requires an inconvenient, sometimes massive adjustment. It hurts business, it limits freedom, and it often has unintended side effects, such as less, more expensive food--and therefore some people will be impacted negatively.
These complexities are what we face every time we try to institute or fight for a new environmental regulation.