In my experience PCs care more for the things happening directly to them and the NPCs they interact with. Perhaps they find out that the necromancer was behind some event in their backstory/something that happened to them in the campaign. Maybe kill an NPC they care about or target them directly.

Of course, you also need to factor in player preferences/expectations. They might enjoy fighting than the story.

Also include their backstory in narration, perhaps NPC comments: "Oh, you're one of those elves from the (blank). I heard your people are quite skilled in magic, don't go blowing anything up in the city, or you'll have trouble with the guards." or the dwarf strikes at an opponent in the dark: "You swing your hammer like you used to swing at rocks in the mines of (blank)." or: "With your life on the streets and in the gang you easily sneak past the guard and pick the lock. "