Because of the crimes committed by Reconstruction governments. Reconstruction officials made it their intent to humiliate Southerners, they wanted to emasculate them, they treated them as traitors (which they were not), and they treated them as whores (think Ben Butler), they in general, treated them like dirt. Reconstruction officials also succeeding in leading nearly ever major city in the former Confederacy into default, and of course, these cities defaulted almost always just as Reconstruction ended, so Redeemers had to clean up the mess the Radicals left.
Now, at the same time, the Reconstruction governments provided a multitude of services to poor whites, they established extensive welfare states. Areas that would later become the backbone of the Populist movement recieved tremendous aid from the Freedman's bureau, which in this state, was more concerned about getting poor white votes for the Reconstructionists then it was with helping blacks. Nevertheless, the crimes of Reconstruction governments were plain and clear for everyone to see. In the early 1870s, political violence broke out in many cities over unionist/confederate lines. I have no qualms in admitting, my family took part in the liberation of 1874, though most of our aid was financial.
Nevertheless, in order to get these liberal governments out of power, Redeemer's played the race card, and they won control of most Southern governments. Now, Redeemers had come to blame all the ills of Reconstruction on black voters, which only reenforced beliefs already held by whites. It was in the period post final liberation (the election of Rutherford Hayes) that the insidious political force known as populism gained steam. In most of the South, populists were virulently anti-black, because as a movement composed of poor whites, it's members competed with blacks for the same jobs, and blacks of course, would do the same work for less pay with less complaints. Seeing this divide in what was left of the Reconstruction party, the Bourbons reached out to black voters, and very quickly, black voters in the majority of Southern states allied with the state's conservative establishment. Poor white labor, understandably became even more angry at blacks. Hence part of the program that was inevitably advanced by the populists involved the implementation of Jim Crow, prime example of this would be the Tillman regime in South Carolina. Populists won control of many states in the 1890s, and, in playing up to their poor white supporters, they began the process of disenfranchising blacks from society, despite the wishes of those in the aristocracy of these various states, who, if not sticking up for blacks out of altruism, they were at least sticking up for them.
Now, all this political history that I have described grew out of the events of Radical Reconstruction, had Reconstruction proceeded in the way proposed by Lincoln/Johnson rather than the radical program of Thaddeus Stevens, then chances are, Jim Crow would have never occured, because there would not have been the kind of post-Reconstruction political turmoil that there was, there wouldn't have been the anti-black backlash that dominated politics after Reconstruction, and issues of class which led to the success of populism could have been blunted from the start. So yes, Thaddeus Stevens bears the lions share of responsibility for Jim Crow.
So if I understand what you're saying, because the Republican government enforced the Constitution and gave blacks a level of political equality that they never enjoyed in the south prior to the rebellion and didn't enjoy immediately after the rebellion, that was all the excuse the south needed to ensure that blacks were denied those right for the 80-plus years following reconstruction?