The most likely reason the justices ruled as they did in JEB v. Alabama ex rel. T.B. (1994) is:
Peremptory challenges on the basis of gender were a violation of the Sixth Amendment.
In this case, the Supreme Court held that using peremptory challenges to exclude jurors based on gender constituted discrimination and violated the Equal Protection Clause of the Fourteenth Amendment. The Court found that such practices were not permissible, paralleling previous rulings that held peremptory challenges based on race were also unconstitutional. The ruling emphasized the importance of a fair and impartial jury, free from discrimination based on immutable characteristics like gender.