“If the apostles picked a replacement for Judas when he died, why didn’t they pick a replacement for James when he was killed?”
We know that the Lord told the Twelve that “in the regeneration” they would “sit upon twelve thrones, judging the twelve tribes of Israel” in the kingdom of heaven (Matt. 19:28). So when Judas betrayed the Lord and took his life, it was necessary that he be replaced, as Peter explained in Acts 1:15-26.
But had the apostles replaced James when he was killed (Acts 12:1,2), there would be thirteen apostles in the resurrection and only twelve thrones on which for them to sit. Remember, James will rise in “the first resurrection” (Rev. 20:5) to assume his throne with the rest of the Twelve, but Judas will not.
This is similar to how Job lost “seven sons and three daughters” (Job 1:2 cf. 1:18,19) and lots of livestock (1:3 cf. 1:14-17). Later, when “the LORD gave Job twice as much as he had before” (42:10), He doubled Job’s original number of animals (42:12) but only gave him an additional “seven sons and three daughters” (42:13). You see, he hadn’t lost his family eternally. They will rise with him in the resurrection, where he will have twice as much family as he had while here on earth. And the Twelve hadn’t lost James eternally either, so there was no need to replace him. With the kingdom program in abeyence, a twelfth apostle would not be needed before the resurrection. source