Why is there so little mention of Scanlon, What We Owe to Each Other, and contractualism?

The most obvious reason is probably aggregation. Scanlonians are among the philosophers most interested in developing non-aggregative theories of beneficence, and EA analyses tend to assume purely aggregative theories of beneficence as a starting point. More simply it could just be that Scanlon is still relatively obscure despite his moment in the sun on the Good Place.

I did read some Scanlon a while back, and recall finding the theory a bit ungrounded compared to utilitarianism (lots of reasonable people being reasonable to one another without a reasonable explanation of "reasonable", which everything rests on).

I think Will named "What We Owe the Future" as a kind of retort to "What We Owe to Each Other" FWIW.

I think the most basic answer is that Scanlon's philosophy doesn't really address the questions the EA community is most interested in, i.e., what are the best opportunities to have a positive impact on the world? What We Owe to Each Other offers a theory of wrongness, which is a very different framing. 

I'm a fan of Scanlon's work, but it has some pretty significant gaps, in my opinion. For example, it doesn't give great guidance on how to think of moral obligations to non-human animals or future generations.

I think you can make a pretty persuasive Scanlonian-style argument for some of the GWWC-style work, global health interventions, etc. But I'm not sure the Scanlonian argument adds all that much to these topics.

I think people could probably get a lot out of reading Scanlon, especially those who want to better understand non-consequentialist approaches to morality. But there are a lot of good and important books to read, and I'm not sure I'd prioritise recommending Scanlon out of all the many possibilities.

I would like to hear more about this. Want to write a post about it?

There have been about 4 papers on future generations and contracualism, including the following and what it responds to: https://www.cser.ac.uk/resources/wrongness-human-extinction/q

I don't get the impression that EAs are particularly motivated by morality. Rather, they are motivated to produce things they see as good. Some moral theories, like contractualism, see producing a lot of good things (within the bounds of our other moral duties) as morally optional. You're not doing wrong by living a normal decent life. It seems perfectly aligned with EA to hold one of those theories and still personally aim to do as much good as possible.

A moral theory is more important in what it tells you you can't do in pursuit of the good. Generally what is practical to do if you're trying to effectively pursue the good and abiding by the standard moral rules of society (e.g. don't steal money to give to charity) go hand in hand, so I would expect to see less discussion of this on the forum. Where they come apart, it is probably a significant reputational risk to discuss them.