Finally someone who admits that there were Constitutional grounds why Davis and his gang were never tried. But, I assume, that you arguement would be that secession was protected by the Constitution and that is, of course, false. Davis wasn't tried because the 14th Amendment had been ratified. Since that prevented the southern leadership from holding elected or appointed office in the Federal government, Chief Justice Chase's opinion was that any trial, conviction, and sentence for treason would violate their 5th Amendment protection against double jeopardy.
The truth is that Andrew Johnson pardoned Davis, just before a 3 judge panel could be assembled to vote on whether or not to indict him. Many wanted him tried, but those who looked into the Union's case started having second thoughts. After the pardon, they quickly lost interest. It was a convenient excuse not to try him, since he would have proven secession to be constitutional.
A voluntary Union is a more Perfect Union.