Well the answer is they were getting that money away from the Southerners, and they were doing it because they controlled Washington DC and could write laws and implement government policies that pried it away from the Southerners.
How many of the first 16 presidents of the United States owned slaves.
What year did the number of free states out number slave states in the Senate
The South was not politically powerless, most of the legislative items passed by the Congress got passed because Southern states voted for them and a Southern President signed the law.
The navigation act of 1817 occurred when the South had considerable power in congress. They didn't grasp at the time how it would work out in practice and be used against their interests in subsequent years.
They may have felt at the time that their political clout would keep it from getting too bad on them, but over time, they found their political clout waning as states moved from supporting them into the political block of the Northern states.
By 1860, they could stop nothing bad against them in congress. I think that when they realized Kansas would be kept from their coalition, that there was no further point to being in a Union which exploited them greatly and maligned them even more greatly.
Who wants to live with people demanding money while calling you horrible things?