To put it as simply as I could, LessWrongers like to quantify stuff. A more specific instance of this is the fact that, since this website started off as the brainchild of an AI researcher, the prevalent intellectual trends will be those with applicability in AI research. Computers work easily with quantifiable data. As such, if you want to instill human morality into an AI, chances are you'll at least consider conceptualizing morality in utilitarian terms.