The question is what argument do you use when faced with this question.? The two most popular arguments go along the lines that if Davis would have stood trial it would have proven that secession was legal. The other argument is that the 14th amendment protected Davis from trial. Davis had already received punishment and to put him on trial again would have proven to be double jeopardy.
Robert Ramirez a trial lawyer from Austin Texas had this to say in a recent interview.
The prosecution was convinced that it’s going to be useless to try Davis. If they lose, it will look really bad for the feds. If they convict him, it will look like retribution rather than a just punishment. Attorney General Evarts solicits a letter from Richard Henry Dana recommending that the case be dismissed. When he finally presents it to the president, Johnson allows the case to be dismissed. Davis is in Europe and the case just fizzles. The people driving the case early on were the people that loved Lincoln like Edwin Stanton and Joseph Holt. But they begin to leave the government, and more people take office who have no interest in prosecuting Davis.
So now it’s here your argument on why the trial never took place.