Part of the problem is the First Past The Post system. Any third party will just bleed votes away from the closest major party. In the case of a party dedicated to protecting the environment, this means votes taken from the Democrats. If the third party grows too powerful, they will split too many votes with the Democrats and Republicans will take control. And once that happens, the environment will be trashed. (Well, trashed even more than it already has been.)

If we switched to Ranked Choice Voting or Approval Voting, you could vote for a third party and still pick a major party candidate as your fallback position. So you could theoretically vote for Jill Stein (ignoring all the other problems with her) and vote for Biden as your second pick (or third or fourth). When Stein failed to get enough votes, your vote won’t be wasted, but would revert to a different choice.