Makarov doesn't seem to have much of a problem using Fairy Law against Phantom Lord so why did he not use it against Tartaros? Even if it couldn't defeat the 9 demon gates, he could at least take out all the minions with it and it would probably at least do some damage to the demon gates. After all, it disabled Phantom Lord's master and he was one of the 10 wizard saints so it seems like he should have used it. It's not even like it would be overkill since all magic was on the line. There also weren't any powerful people around who could immediately counter it like with Hades so it wouldn't pose much of a danger to the rest of the guild.
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.
The other reason is that phantom lord attacked the guild hurt team shadowgear kidnapped Lucy and put makarovs family through hell this pushed makarov to righteous fury kinda like when natsu is fighting eragor and happy says gray could do better than him a wizard's magic power is connected to their emotions to some extent and makarov was also friends with hades in the past so perhaps he still felt some compassion for hades and knew that fairy law wouldn't have any effect if he had the compassion for his enemy in his heart