I didn't know about this code of conduct until just now when I saw this article, so I don't know for sure why people are so against it.
Part of the problem may be that there have a been a few high profile cases (posted to HN, etc.) where a bunch of people unrelated to a project (not committers, not forkers, not users) have come in and started big commotions over committer behavior outside of the project (i.e. on their personal twitter account having nothing to do with the project), or have demanded changes like "he" be replaced with "they" or "he/she" in the documentation. Basically, a bunch of political stuff that doesn't have perceived technical value for the project.
It could be that a GitHub endorsed "code of conduct" would give those people more ammunition against projects, and I think that worries people. Most GitHub users aren't big OSS projects, they're just using it to host their hobby projects, they're not out to make political statements.