Question

Why did the Union struggle to take Vicksburg from the Confederacy?

Answer 1

Vicksburg was the last Confederate stronghold on the Mississippi. The US wanted to control the entire Mississippi River and cut the Confedrate States off from Texas and their supply of beef for troops.
Grant made several attempts to take the stronghold. It was very well defended and impossible to take by water. Grant laid siege and made several impressive maneuvers during the decisive victory.