Why did Germany refit the Pz. IV with a long-barreled 75-mm gun?

Germany refitted the Pz. IV with a long-barreled, high-velocity 75-mm gun specifically to counter the threat posed by the Soviet T-34 tank.12

When German forces first encountered the T-34 in late 1941, the original short-barreled gun of the Pz. IV proved inadequate against the Soviet tank's armor. By upgrading to a long-barreled, high-velocity weapon, the Pz. IV gained the necessary penetrating power to engage the T-34 on nearly equal terms. This modification allowed the Pz. IV to remain a formidable component of Germany's panzer divisions through the remainder of World War II.21