There was a reason for this....the North would have had to explain a lot of things leading up to the South deciding to secede.....Lincoln knew this would further antagonize the south so it was decided to forgive and forget(so to speak)!!!! Lincoln insisted that Grant go easy on the south for this reason!!!
Lincoln was gone long before the idea of treason trails could have come up. But Lincoln, for whatever reason, really wanted to put the nation back together again as easily and painlessly as possible. He always said the war was being fought to preserve the Union—that was his primary goal.
I’ve always thought that a really interesting “what if” question in history is if Lincoln hadn’t been assassinated. He wanted the nation reunited. The radical Republicans in Congress wanted the South punished. While the war was going on, Lincoln could use his authority as CinC to get things done. Once the war was over, Congress was ready to take charge. The fight between Lincoln and Congress wouldn’t have been pretty.