The scope of interpretation by the park was expanded by Congress in 2006. There weren't USCT at the siege, but there were USCT in battles during the campaign and afterward garrisoning the city and nearby points.
The capture of Vicksburg opened the river and separated the Confederacy from many key resources needed to prosecute the war: Texas beef and nitrates, Louisiana salt and sugar. But the primary advantage gained was access to the manpower of the region for military recruiting and cotton production in the form of several hundred thousand freed slaves.
Given that Vicksburg was the first major military operation with the express goal of freeing the slaves and recruiting them into the ranks of the federal army, it seems a given that slavery and the USCT would be a major component of the interpretation.