Because then, as now, politicians act differently in DC than they do at home. Politicians tend to become more in favor of big government in DC, regardless of where they're from. It's no different now.
To me that point touches on one prevalent misconception held by fans of the Confederacy. There was a federal government in Richmond too. And given the fact that it had to support a society based on suppressing a large segment of the residents, I believe it would have developed into something much more oppressive than anything we've ever seen out of Washington DC.