In Avengers: Infinity War, after the passage of Thor, Groot and Rocket to Nidavellir, Eitri has nothing left to lose, his planet is dead and he can no longer forge weapons by himself. Given that he is huge and that he can defend himself or at least help, why did Eitri not join the Battle of Wakanda?
TL;DR: Eitri is not a fighter but a blacksmith and he wouldn't really be of much help in the battle. It is better to keep him, and his knowledge, safely out of the way.
He wouldn't really have been much help. Yes, Eitri can defend himself but when he attacked Thor he was mainly using the element of surprise and his giant size to his advantage. He would lose one of those things in the battle and the other counts for nought against multiple opponents overrunning you.
Eitri is also quite a slow mover, for his size, and pretty clumsy (especially since what Thanos did to his hands). All these things count against you in a battle. Add to this that Eitri is not a fighter and it's understandable he doesn't want to fight.
It is further reinforced that Eitri, and the dwarves in general, are not fighters as defending Nidavellir was the job of Asgard not themselves; they are just smiths.
Eitri: You were supposed to protect us. Asgard was supposed to protect us!
Avengers: Infinity War
It's worth pointing out again that his hands are encased in metal and so he can't get them out and they are pretty much unusable. His sole power in a fight would then have to come from blunt force punching rather than being able to use a weapon. This puts him at a further disadvantage.
Eitri is best kept out of the way where he can potentially make more weapons for them should they need him to and also potentially another Gauntlet. The knowledge in Eitri's head is worth more than losing him over some fight where he wouldn't help that much anyway.
Eitri: 300 dwarves lived on this ring. I thought if I did what he asked, they'd be safe. I made what he wanted. A device capable of harnessing the power of the stones. Then he killed everyone anyway. All except me. "Your life is yours." he said. "But your hands are mine alone."
Thor: Eitri, this isn't about your hands. Every weapon you've designed, every axe, hammer, sword...It's all inside your head. Now I know all hope feels lost. Trust me, I know. But together, you and I, we can kill Thanos.
Avengers: Infinity War