I think research did go into this, because the FIA did become concerned about the weight of crash helmets with young drivers, and produced new standards in 2007. One of the guys on the project Dr Terry Trammell is a spinal injuries specialist now with IndyCar. Maybe he would be a good guy to contact with respect to the subject.
My speculation is that they saw the additional weight of an adult crash helmet was a problem for kids, such that it was worth reducing the mass of the helmet and therefore impact protection, to protect from added risk to spinal injury.
Therefore they likely looked closely at spinal injury risk and efficacy of additional neck protection, particularly for young kids. My further speculation is they rejected the idea.
It would be really cool to find out more, maybe someone here has contacts at Indycar and can reach Dr Trammell somehow for comment.