Why does the Bible say that the wicked will be destroyed with unquenchable fire?

Unquenchable fire is fire that cannot be put out, but which goes out when it has turned everything to ashes.  Jeremiah 17:27 says Jerusalem was to be destroyed with unquenchable fire, and in 2 Chronicles 36:19-21 the Bible says this fire burned the city "to fulfill the word of the Lord by the mouth of Jeremiah" and left it desolate.  Yet we know this fire went out, because Jerusalem is not burning today.

To quench means to extinguish or put out.  No one will be able to put out the fire of hell.  That is the strange fire of God.  No one will be able to escape from it by extinguishing it.  Isaiah says of that fire, "Behold, they shall be as stubble; the fire shall burn them; they shall not deliver themselves from the power of the flame: there shall not be a coal to warm at, nor a fire to sit before it" (Isaiah 47:14).  After it has accomplished its work of destruction, that fire will go out.  No one can deliver themselves from its flame by putting it out, but finally not a coal will be left.  So say the Scriptures.