I don't know if it's anything specific... classic utilitarianism is the most common form of consequentialism espoused on Lesswrong, I think, so it could be as simple as "the most commonly encountered member of a category is assumed to represent the whole category".

It could also be because utilitarianism was the first (?) form of consequentialism to be put forth by philosophers. Certainly it predates some of the more esoteric forms of consequentialism. I'm pretty sure it's also got more famous philosophers defending it, by rather a large margin, than any other form of consequentialism.