Because Charleston was the second largest port on the american east coast. The revenue collected there was an important part of the federal budget. Lincoln even said it about who would make up the revenue loss if South Carolina seceeded. The southerners saw those large numbers of troops in the 1850's for what they were, a threat.
The same threat that Andrew Jackson and Winfield Scott had used during the Nullification Crisis in 1832 and the same threat that Major Robert Anderson tried at Fort Sumter. It was the same repeating pattern. A the Sword of Damocles constantly hanging over the people of Charleston. A one of the many reasons that the south did not trust the northerners or the Government.