Environmentalists should be libertarians because free markets will in most cases better satisfy their desires - saving the whales, preserving forests, stopping animal cruelty, etc - than will socialist alternatives.
Libertarians qua libertarians do not have a view on whether any particular environmentalist goal is worth it or not. We just say let's have a free market so that the views of everyone are encapsulated in free market prices, so that the amount of resources devoted to particular and environmental causes generally is precisely the amount of resources that the individuals in society are willing to devote to them.
The alternative is having the politicians guessing this and using highly inefficient (and unethical) socialist techniques to try to achieve environmentalist goals. Also, there are the usual public choice problems associated with socialism and democracy in particular: pandering to special interests, bribery, corruption, short-term thinking, waste, etc. These are widespread problems when it comes to state environmental policy-making.