The best explanation for why the influence of the U.S. Constitution and Bill of Rights has lessened in recent decades would be:
The U.S. Bill of Rights, while revolutionary when it was written, increasingly seems too limited in comparison to newer rights documents.
This response reflects the idea that as societies evolve and new rights issues emerge, the original framework of the Bill of Rights may seem inadequate or outdated relative to contemporary understandings of rights and freedoms as seen in other documents and movements around the world.