Migrant workers are an important resource for seasonal farmers, and there is a significant migrant population here in the United States. If the US had failed to sign this treaty, they would probably develop a bad international reputation. This would not help America's diplomatic relations with other countries in the least.
The UN has no rights to dictate how migrant workers should be treated on the world stage because doing so implictly gives the UN authority over the immigration policies of countries, something that they most definitely don't have. The US should be less beholden to international pressure and decide its own policies domestically.