Several possible reasons. Here is the first, and most obvious.
It is already established that the Court will maintain an unconstitutional ruling if to overturn it will disrupt things beyond their tolerance.
As Justice Sandra Day O'Conner, writing for the majority in the case of PLANNED PARENTHOOD v. CASEY, "Stare Decisis." (Which means Shut the F*** up! We have made our decision!)
Given that it is axiomatic that "Constitutionality" is not the Supreme goal of the Supreme Court, it requires no imagination to wonder why the Judges might run screaming in terror from the very thought of turning out the first "Black" Precedent.
If they thought that ignoring the case would result in massive quantities of people burning down cities and killing many thousands, they would very likely have taken the issue up.
But you go on thinking that this is all about law and constitutionality.