After the destruction of lives and gratuitous devastation at the command of General Sherman, the South was ripe for corruption from the North and those politicians who sought to virtually punish the South for its "rebellion." Greedy and unconscionable carpetbaggers came to the tragically defeated Southern states and exploited the indigent landowners purchasing their plantations sometimes just for the price of the taxes, and making other profitable financial gains. Others with aspirations of political gain promised freed, destitute, illiterate, and vulnerable former slaves, who had been released with virtually no place to go, the false "forty acres and a mule" and enrolled them into the Republican party. Those called "scalawags" were the white men who also joined this new Republican party of the South, hoping for personal gain as well. Their intentions were not noble, to say the least. For, while Presidents Lincoln and Johnson undertook policies that were designed to return the South as quickly as possible to some sort of normalcy,
...the Radical Republicans used Congress to block the president, impose harsh terms, and upgrade the rights of the Freedmen (the ex-slaves).
Wielding such power, the "Radical Republicans" caused the Southerners who had participated in the war to become disenfranchised citizens. However, the Democrats were also culpable of exploitation as evinced in the writing of Southern historians. For instance, one writes,
On economic matters scalawags and Democrats eagerly sought aid for economic development of projects in which they had an economic stake, and they exhibited few scruples in the methods used to push beneficial financial legislation through the state legislature. The quality of the bookkeeping habits of both Republicans and Democrats was equally notorious.
It is hard to say exactly why any era is the way it is, but we can speculate. We can speculate that this era was so prone to corruption because it was the time when business was just starting to get really big.
During this period, America was industrializing. Businesses were starting to consolidate and become really big. This meant that there was a lot of money to be made in business. At the same time, there was coming to be more of a connection between business and politics. The Civil War had made some of these connections and more had been made as the government subsidized railroad construction.
This meant that there was more money at stake and there was the idea that government could help to decide who would get that money. This led, fairly naturally, to a situation in which corruption could flourish.