Why did Makarov not use Fairy Law against Tartaros?
Fairy Law didn't work that simple. Fairy Law consumes magic power as much as needed to defeat whoever the caster deemed as enemies. The sheer number of the enemies present there would made Fairy Law a sucidial move. This is evident in the final battle where Fairy Tail went to fight the Alvarez Empire. Makarov died (or at least knocked unconscious) there after casting Fairy Law to reduce the number of enemy troops.