It seems to have been more a result of Grant's tactics of attrition in 1864. He wanted to outflank and bleed the confederates dry, and the Confederates responded by digging in to make up for their numerical weakness with earthworks. Lives meant more to the Confederates than the Union at that point, so they naturally sought to protect that dwindling asset.

On another level, being stuck to the defensive in front of a strategic target which you cannot give to the enemy gives more incentive to invest in powerful static defenses. There were trenches around Vicksburg as well when Grant laid siege there, and which proved effective at stopping an assault by Sherman's men. The tactic just reached a larger scale in the east, and had more attention focused on it as that was the main theater of the war in the eyes of the Northern public.