Because he just flunked Constitutional History 101. As the Harlan Institute's Josh Blackman reports:
John Adams was in Europe during the drafting of the Constitution, and did not return to the United States until 1788.
Listen to Justice Breyer's interview on NPR at 1:20. Terry Gross asks about his view of the Constitution. Justice Breyer replied:
"I can say what I think, I think we are following an intention by people who wrote this document [the Constitution], Madison, Adams, Washington…."