Reason it failed is the reason that most (or many, at least) international treaties fail. The US does not gain from them. Other countries gain because they get to be included in some larger club of cooperation when they sign them. They get benefits from them. The US, on the other hand, has no need to be part of that club. It's more likely to harm us.
If we already have an ADA within our country, and we're answerable to ourselves (our SCOTUS), then why subject ourselves to international law, also?