You do not appear to have read Washington's Ruminations on the subject. It is clear that he undergoes a transformation toward's the end of his life.
Lincoln went from accepting slavery to supporting abolition and voting rights for some African-Americans.
Power. Both things can be explained by a desire to control power. Just after the civil war, voting rights were suspended for whites in many states. Only blacks could vote. Of course they voted Republican. What sort of effect do you suppose this had on power?
You need more cynicism. You need to stop looking at things through rose colored glasses. Look for evil reasons, and you won't have any difficulty seeing them.
Let's jump to 1963 to the 24th amendment. We could simply accept the claim that North Eastern Liberal Republicans pushed the 24th amendment because they felt so strongly about making sure black people could vote, and so they supported it as a moral issue. (What are North Eastern Liberal Democrats doing about illegal aliens?)
On the other hand, adding 10 million or so new voters that can be counted on to reliably vote for their party, well that simply guarantees their control of the Congress and the Presidency. In other words, they profit from this move.
Lyndon B. Johnson can in no wise be considered a paragon of virtue, and he gave no previous indication that he gave a rat's @$$ about the condition of blacks in America, but suddenly in 1964 he decided that huge sums of federal money needed to be spent to help lift blacks out of poverty.
Say a lot of things about Johnson, but politically he was a canny bastard. He knew exactly what he was doing, and his plan worked magnificently.
He flipped those 10 million votes the Republicans thought they were going to get, and Johnson turned them reliably Democrat.
Now I think you will agree with me that Johnson's moves were obviously cynical ploys to gain more power. You may have a harder time thinking of the originally Republican efforts to pass the 24th amendment as a cynical ploy to gain power, but the facts do appear to support that interpretation of events.
Johnson simply outsmarted them. He turned their own plan against them.
Now is it too far fetched to assume that political operatives in the 1860s were as canny and smart as political operatives in the 1960s?
Are we to assume that these people are only motivated by morality and not by power, and that it is just a coincidence that their pursuit of morality is concurrently operating to increase their power?
I don't believe in the good will and morality of any politicians anymore, and I have come to the conclusion that it has always been thus.
You need more truth telling.