For the same reason that Luther wanted to remove the book from the Bible. It states this:
Ye see then how that by works a man is justified, and not by faith only.
Justified before whom? That is what is overlooked. Justification holds only one meaning to some. But, that is not the case.
What I am about to share is applying how the word was used, in the context of what was going on in Jerusalem at that time. Revealing what motivated James to use the word 'justification.'
James' perspective:
Jewish believers in Jerusalem were being boycotted by religious unbelieving Jews. Businesses of these believers were being ruined. Believers were beginning to starve and needing help. So, when a brother in need comes to you, asking for help? Do you just tell him?.. "Be well and may God bless?" Or, do you do something to actually help? Works in helping them.
OK believers who are still feel secure. Sure faith saves us. OK... So, you believe in God and that saves you from this mess we are in? Good for you! Even demons believe there is one God and shudder! Well, you? You say your saved? Why? Because you say the same thing as demons?
You want to be
justified in your claim of being saved? Let us see the difference between you and a demon. How? It will be by your faith that your claim to salvation that will be
justified before others to see by your works!
How can you say you believe and are saved? What justifies you saying that? Are demons saved?
Your works will end up being your justification for making such a claim. Otherwise... even a demon can say he believes in God. Even unbelieving Jews who boycott Jewish believers say that believe in one God. So?
Where is the justification that sets the unbeliever Jew (who say there is one God) apart from the believer in Christ? Your works will save you by God honoring your attitude to help your persecuted Jewish brothers whom unbelieving Jews wish to destroy.
Try to save your life? You will lose it. That is what Jesus said. Jewish believers seeing their brothers in need, were not wanting to make waves by helping a brother who was being boycotted. So they would be simply saying "be blessed" and keep them moving along without drawing attention to themselves.
In the mean while, that would not save them. (they were trying to save their life). They had to lose it to save it, because God would honor their "works." Just like Abraham was ready to lose Isaac on the alter if need be..... It was a real lesson in actually putting their very lives into the hands of God. Their works were their "offering of Isaac." In doing so, i justified their faith in their claim for it being real. For the unbelieving Jews in Jerusalem were like ANTIFA going around wishing to destroy believers. They were putting their lives on the line to help brothers in need.
That may help by better understanding the background as to what led to James to use the term justification. The word holds several meanings. Not only one like Luther stumbled over.
What I said probably needs more work. But, its a beginning to better understanding what was behind what James was getting at. Hope that helps.
grace and peace...