The most likely reason the justices ruled as they did in JEB v. Alabama ex rel. T.B. (1994) is that 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 their gender constituted unlawful discrimination and violated the Equal Protection Clause of the Fourteenth Amendment. The court drew parallels to its previous decision in Batson v. Kentucky, which addressed the use of peremptory challenges based on race, establishing that such practices are impermissible. Although the question refers to the Sixth Amendment, the ruling primarily pertained to the Equal Protection Clause regarding discriminatory practices in jury selection.