because judgment without mercy will be shown to anyone who has not been merciful. Mercy triumphs over judgment!
To jump right to the point... (like James likes to do)
If you show judgment without mercy, there is a high probability that you have not yet understood the mercy of God or have never recognized your encounters with God's mercy. To put it bluntly, if you don't show mercy, then maybe you aren't even saved.
This is a perfect lead-in to the next set of verses.