Why does lincoln feel his second inaugural address should be shorter than his first?

He believed that there's no need to say so much because he already said everything that was needed and people knew that and knew what the political currents of the time were. He believed that those who believed in his cause were already behind him and those who weren't will probably not be suddenly swayed out of nowhere, so everyone just needed to stay firm in their beliefs and things will go well.

Best thing to do, reread whatever you are writing on, and highlight things that are scientific or anecdotal. then pick which ever is easiest for you