This, of course, is an incredibly complicated question, and there is no way I can do justice to the equally complicated answer in 200 words, so I'll simply do a brief overview. Rather than simply consulting this response, I'd recommend looking at articles for historical background and context and then consulting the books Reconstruction: America's Unfinished Revolution (Eric Foner), Stony the Road (Gates), and Black Reconstruction in America (Du Bois). Britannica defines Reconstruction as
the period (1865–77) that followed the American Civil War and during which attempts were made to redress the inequities of slavery and its political, social, and economic legacy.
This was an incredibly divisive time in American history, with the South defeated and in shambles, tens and thousands of slaves finally free, and a destructive war having just concluded. Andrew Johnson became president after Lincoln's assassination, and many historians would argue that he was not prepared for the challenge. He is often ranked as one of our worst presidents. I think we need to separate Johnson from Reconstruction. Reconstruction was an ambitious and necessary plan to both bring the defeated South back into the Union and, perhaps more importantly, enfranchise newly freed slaves and try to fully integrate them into America after centuries of slavery and mistreatment. Johnson was more interested in restoring the South than in the rights of black citizens, which led to numerous political clashes. Another key reason that Reconstruction failed, aside from a lack of political will and resources, is the unrepentant racism of Southerners who refused to accept blacks as equal. This is a very simplistic look at a key part of American history.