The DNA seemed like it should crack the case, in that if they know who baby John's parents are then surely they could find out from them what happened, but that has not happened. There have been no confessions etc.
I would guess that there is an issue around connecting the parents to the death. This is pure conjecture on my part, but something like "we gave the baby to Mr X who said he could arrange for a private adoption in America. We have lived the last 40 years assuming that was the case, until this DNA result. We had no idea that baby John was our baby, and Mr X is long dead."
Now, you or I might find it very suspicious. We might think that surely if a dead baby was found a few weeks after your baby had supposedly been spirited off to America that it might occur to you that that was your baby. Would that be enough for a murder charge though? Probably not.
Again, this is in no way based on anything I have heard or read or anything. Pure speculation.