Well, for starters, I think you should re-examine your approach to this question. Whether you are pursuing this topic as a result of a school project/assignment, or even just for your own mental satisfaction, one of the cardinal rules of any historical investigation is to approach a topic objectively in pursuit of a truth(s), not an "answer," and certainly not an answer pre-determined from the start. Any person can cobble together a random assembly of facts to support a foregone conclusion: a historian looks at evidence, both primary and secondary, to get at the truth of a person, topic, or event. Thus, while your intentions here might be good, your line of questioning threatens to derail your investigation before it even begins. This is the slippery slope that leads to "research" that is laughable at best (i.e., Ancient Aliens), and morally repugnant at worst (Holocaust denial).

That aside, to get at the heart of what you're asking, I think it is important to define your terms, here. What does "failure" represent as it concerns the very broad topic of U.S. Reconstruction? On the one hand, while Jim Crow laws, racial violence, and a general quashing of African-American civil rights could absolutely be seen as harbingers of Reconstruction's failure, the fact that there wasn't any serious flare up of succession tendencies, and that there was a peaceful reintroduction of the southern states back into the Union could rightly be seen as a success. Wins and losses could be viewed north of the Mason Dixon as well. Although African-American migration north into cities like Chicago and Detroit could be viewed as a positive development as it concerns upward urban mobility, the squalid conditions these families lived in and the racial violence they experienced in these northern cities could also rightly be called a failure.

So if we take all of these failures together, and are looking to assign blame, where do we look? Certainly, it starts with John Wilkes Booth, who murdered the one man (Lincoln) that might have been able to hold the coalition of moderate and radical republicans together long enough to institute reasonable, effective Reconstruction policies. This veers into history-what-if territory, however, so next we're forced to look at President Johnson, who incorrectly took the temperature of men like Sherman and Grant, who would not be bullied into abandoning the south to the Confeder-...ah hem, Democrat politicians looking to institute a pre-Civil War social contract between themselves and the African-Americans down there. And while the radical republicans certainly towed a hard (nay, one might say impractical) line, their intentions were mostly good. Grant's administration largely sided with them during his first term, and is often not given enough credit for its ambitious civil rights planks, yet even that administration could not keep the south from descending into a brutal cycle of violence and state-sponsored discrimination.

Which is all to say that this topic is a very complicated one, and not easily explained away by a magic bullet culprit like the radical republicans (or anyone else, for that matter, except perhaps the racist Confederate-sympathizing jerks and their sympathetic northern counterparts who felt threatened by racial equality). There's a lot more you could talk about, from the Enforcement Acts to Redeemers, but really, this topic is far to broad and nuanced to tackle as presented here, with OP's question.

[Sources: Jay Winik, 'April, 1865'; Bruce Catton, 'A Stillness at Appomattox'; Ronald C. White, 'American Ulysses'; Allen Guelzo, 'Fateful Lightning']