Because he just called Roe v. Wade an unsupportable decision. He does day that it should remain as precedent, underthe principle of stare decisis, but that the decision is not based, “in either the text of the Constitution or its own precedents.”
First, this analysis is just plain wrong, the precedents of Griswold v. Connecticut and Eisenstadt v. Baird, which prohibited states banning contraception on the basis of privacy rights, and which he studiously, ignores is clearly on the path to Roe.
Second is that his embrace of stare decisis would invalidate rown v. Board of Education, the national minimum wage, and the National Labor Relations act.
For Barack Obama to continue to have him as an adviser, or to consider him as a member of the cabinet, or as senior Department of Justice appointment would be a betrayal of his supporters and the Democratic party.
H/T TalkLeft.