Fortunately this question is addressed by this post in the FAQ by /u/The_Alaskan

https://www.reddit.com/r/AskHistorians/comments/5hh6if/why_wasnt_the_history_of_the_us_civil_war_written/db0fje7/

I have a follow up question:

Were northerners more willing to accept or put up with the Lost Cause ideology being propagated by the south after the war in order to facilitate the social and political reunification of the north and south? (i.e. by allowing the South to "lose with honor" and therefore keep some respect and feel like they could return to the table as equals). If so, was this willingness a factor that helped lead to the Lost Cause becoming such a prevalent and accepted idea?

I'd like to point out that the idea that 'history is written by the victors' is not that great, and recommend a few good posts here on AH about it:

• In what ways is the "winners write history" trope true or false?
• A famous quote says that history is written by the victors. In your field of study, what is the largest attempt to paper over an event or movement? And how did the powers-that-were try to sequester the information?
• What are some ways history has been written by the victor?

The Top comments by /u/Tiako tackle the issue.