To understand what the Rules are, in legal terms, we have to start with what they arent. The truth is, the Rules are not legally binding on States. To be legally-binding, they would have to be part of an international convention. A convention is a multi-lateral treaty in which a number of countries mutually agree to do certain things. A State party to a convention accepts all of the obligations included in the treaty and agrees to make them part of national law and practice. There is usually a formal mechanism to monitor compliance and if any State fails to implement its obligations, the other parties to the treaty can take action to sanction the non-complying State.
Most of the basic human rights instruments, like the Convention on the Rights of the Child or the on the Elimination of All Forms of Discrimination Against Women are like this.
There was an effort before 1991 to create a convention on the rights of persons with disabilities, but there was little support among governments for this. Some said that the rights of persons with disabilities were already protected by other human rights treaties. Others said that they could not accept the obligations that a convention might bring.
The idea of having such a convention is still being advocated, especially by organizations of persons with disabilities, but governments have mostly not responded yet.
So the Standard Rules remain the main international expression of what governments should do to provide equality to their citizens with disabilities.