So Just Why Has the USA not Signed?

So just why hasn't the USA signed the UN Convention on the Rights of Persons with Disabilities?

Guiding Principles of the Convention
There are eight guiding principles that underlie the Convention and each one of its specific articles:

-Respect for inherent dignity, individual autonomy including the freedom to make one's own choices, and independence of persons
-Non-discrimination
-Full and effective participation and inclusion in society
-Respect for difference and acceptance of persons with disabilities as part of human diversity and humanity
-Equality of opportunity
-Accessibility
-Equality between men and women
-Respect for the evolving capacities of children with disabilities and respect for the right of children with disabilities to preserve their identities