One of the principles of effective altruism is finding causes which are important, tractable, and neglected. Climate change is definitely not neglected - you could certainly make the case that we ought to be doing more, but not that it's ignored.

Research into x-risks from AI, though, remains an extremely niche field - its total funding is probably several orders of magnitude less than the amount spent on climate change. 80,000 Hours care about effectiveness on the margin, which is why their list of priorities looks the way it does.