It is because the initial investigators (the local sheriff) got tunnel vision on "the usual suspects" and didn't follow up more likely alternatives based on the evidence that came up. They cherry picked information to try to make a case against "the usual suspects" even though it became increasingly clear that it wasn't them, and ignored the information that pointed in other directions. Wearing blinkers is never a good idea for investigators, but in those small towns unfortunately it is far too common. And now those other trails will have clouded over and gone cold with time (certainly by the time other agencies took over the investigation). So I don't think the case will ever be solved now, short of a confession.
