There are two main reasons why the US might have taken these actions. Both have to do with the desire on the part of the US to protect its national interests.
First, the US tends to be somewhat reluctant to give up its sovereignty by allowing other countries or international bodies to dictate what it can or cannot do. As the world’s only superpower, the US has become accustomed to having a great deal of freedom of action and does not want to give any of that freedom up. Therefore, it would be reluctant to sign these accords, particularly if they imposed strong limits on the US.
Second, and perhaps more importantly, the US is very concerned that these agreements would be harmful to its economic interests. The US fears that these accords would require it to take steps to lessen carbon emissions that would raise the costs incurred by its companies. The US also fears that the accords will exempt developing countries from taking these same steps. If this were to happen, the US feels, the other countries would come to have an advantage because they could continue to emit greenhouse gasses while the US could not.
For these reasons, the US has been wary of signing these agreements.