Why did Stephen Douglas's atempts to shove aside the conflict over slavery fail?

Douglas failed to realize the enormity of the slave issue as well as opposition of slavery in the north. He opened the issue of letting states decide for themselves whether they should allow slavery. After slavery was voted in, a bloody insurrection between pro and anti slavery groups came about in Kansas. Douglas denounced the use of his rhetoric for political purposes and denounced President Buchanan who recognized Kansas as a slave state.