So that His kings will have glory by searching it out.
Consider three English translations for Proverbs 25:2 to learn this “reason”.
New Living Translation
It is God’s privilege to conceal things and the king’s privilege to discover them.
English Standard Version
It is the glory of God to conceal things, but the glory of kings is to search things out.
New American Standard Bible
It is the glory of God to conceal a matter, But the glory of kings is to search out a matter.