Thanks for posting the link. The charges were dropped in 1869. Prosecutors didn't have a case.
The prosecutors had an excellent case. Charges were dropped, over the objections of the prosecuting attorney and one of the two judges who would have tried Davis, because Chief Justice Chase made it clear that in his view convicting Davis of treason would constitute a violation of his 5th Amendment rights since the 14th Amendment had already levied punishment for his actions in the rebellion, and he would overturn any conviction on those grounds.