I was hoping to see some more answers by now, but seeing none I'll provide some initial points that I expect people who are more familiar with the field could flesh out much better than I. I'm not familiar with the full case for biodiversity, but just based on generic reasoning about cause areas in general, I'd suspect some of the major reasons EAs do not seem to consider it as important as factory farming, wild animal welfare, or even climate change (to name a few environmentally-oriented cause areas) include:
- It's unclear how significant the extrinsic, welfare-oriented value of biodiversity even is. Put most bluntly, "if some random frog species goes extinct, so what?" I understand there are arguments like "what if this species proves to be very useful for e.g., pharmaceuticals or the broader ecosystem", but I'd want to see some quantification and comparison with other cause areas. Is it worse for the last 100 fish of some random species to die (=> extinction) than for 100 chickens to suffer factory farming conditions? Maybe, but I could also definitely make the case for the opposite conclusion. And as a few other answers/comments pointed out, it definitely seems to be much smaller in significance than human X-risks.
- (On a smaller note since I am not very familiar with this field, I'll add that it's a bit difficult to understand what all of the charts, figures, and bullet points on the link you shared amount to. Lots of things may qualify as endangered, but how many things actually go extinct? How is this impacting the environment? It's all just much more difficult to neatly quantify the situation than it is to put a probability estimate on X-risk, preventable mortality estimate for malaria, number of animals in factory farming conditions, etc.)
- It's unclear whether there are cost-effective interventions (in conjunction with point 4): if you would like to propose/list some, that would help, but if it's the case that vast numbers of these species are going extinct/becoming endangered due to a wide number of environmental changes, it seems more difficult to identify highly cost-effective interventions (in contrast to, e.g., bed nets for malaria, surgery for trachoma).
- Some (non-EA) organizations are already working on the field of biodiversity and conservation. This certainly doesn't mean that it's already fully saturated, but in order to determine whether there are cost-effective interventions that need funding, one must also consider what kinds of efforts/funding already exist.
In the end, this isn't to say that biodiversity is necessarily "unimportant" and completely intractable, but given the combined issues surrounding comparative importance, tractability, and neglectedness I don't know if it's really an area where EA would have a comparative advantage in relative to e.g., AI safety/alignment (which itself may prove very impactful for the environment and many other problems), cost-effective health interventions (e.g., bed nets, deworming), and general long-termism. I would be willing to read an argument for why it is overly neglected, but I haven't seen that argument made yet.
For the second point, I think it's a controversial position whether wild animals have net negative lives, and I think the field overall (that is people, working on wild animal welfare) does not have a strong position on it.