republicans in congress spent more time prosecuting andrew johnson than former confederate leaders. lots of reasons for that, but the biggest was that virtually everything, including trying davis, stevens, et al, for treason, took a back seat to restoring the union, establishing a functioning and loyal government in the states that were formerly in rebellion, and grappling with what to do with 4 million freed slaves — soon to be 4 million brand new citizens after the passage of the 14th amendment.
Trying Davis for treason wouldn’t have been hard to do but it wasn’t done. The reason for that is because the PTB all knew Davis did not engage in treason as it was understood in 1860. The following quote is from a speech by Lincoln in 1846.
“Any people anywhere, being inclined and having the power, have the right to rise up, and shake off the existing government, and form a new one that suits them better. This is a most valuable,—most sacred right—a right, which we hope and believe, is to liberate the world. Nor is this right confined to cases in which the whole people of an existing government, may choose to exercise it. Any portion of such people that can, may revolutionize, and make their own, of so much of the territory as they inhabit. More than this, a majority of any portion of such people may revolutionize, putting down a minority, intermingled with, or near about them, who may oppose their movement.”
Of course Lincoln sang a much different tune in 1861. My point is that the illegality of secession was not a unanimously held view before the war or even after in 1865. I think Davis stood a good chance of winning a treason trial and that is why he wasn’t charged. He wanted a trial but was denied it.