My roommate is the chapter organizer for our city's Humane League Changemakers, inspired down that path because of involvement with effective altruism and having gone to previous EA Globals. He got a form letter rejection from EA Global saying he should learn more about EA and reapply in the future. Make it make sense.
This is cribbing a bit from Samzdat's summary of Hoffer's "The True Believer", but in brief: I'm skeptical of pushes to make EA more normal because normal people don't join social movements or intentional communities. Alternative lifestyles like polyamory, atheism and veganism are already largely drawing from people who are alienated from mainstream society enough to start doing things differently. High tolerance for weirdness + a desire to sacrifice in favor of the cause enables abuse in EA, sure. It also enables abuse and grift in your local anarchist co-op, your local BLM circle, probably even your local fusion dance scene. Joining a demanding community or cause is something that is most appealing to people who are already having trouble fitting into society. That fact creates a lot of problems in any kind of intentional community or cause--I will always be the first to admit this. But not tolerating weirdness probably means paying the price of not really having a movement at all.
Partially the answer to this question is that biodiversity protection involves a wide variety of interventions, some presumably effective, some not, and across all those interventions it is not clear how neglected the cause area actually is.However, a bigger reason might be the inherent conflict between traditional environmental protection, focusing on biodiversity, and the animal welfare/liberation movement. The latter is a lot more prevalent in effective altruism, through organizations like Animal Charity Evaluators and Wild Animal Initiative. There are some sources I can recommend to help clarify the nature of this conflict, starting with "A Triangular Affair" by J Baird Callicott, and "In Nature's Interests?" by Gary Varner, but briefly: if wild animals are moral patients with rights, it is not clear that their status as this-or-that-endangered-species matters very much, and vice versa. In EA, we tend to focus on the experience of animals as a neglected source of suffering, and that does trade off against biodiversity concerns.Finally, the philosophical case for why we should value biodiversity isn't clear-cut, although I certainly share your intuitions about it! This is an essay from a biologist in the community about what might be valuable about biodiversity and how mainstream environmentalism might be getting it wrong: https://eukaryotewritesblog.com/2018/05/27/biodiversity-for-heretics/