First, a correction. Most "gun deaths" are not caused by "legal weapons" but by criminals shooting victims. The police and armed citizens rarely shoot people by comparison. The problems isn't "illegal guns" but instead criminals using weapons. Criminals also use weapons other than firearms.
"Smart guns" haven't caught on because of a failure of the inventors to understand the role that firearms play for police and armed citizens. Firearms are safety equipment, similar in many respects to a fire extinguisher. In an emergency they are needed RIGHT NOW and must function WITHOUT FAIL, because if they do not function then human life is threatened.
So far none of the biometric gizmos can provide that 100% guarantee. Simple as that.