Why is the soul not immortal?

First it is important to understand exactly what the soul is:

"Soul", in the original-language terms ("Ne´phesh" in Hebrew and "psy·khe´" in Greek) means a person, an animal, or the life that a person or an animal enjoys.

The common misconception is that a “soul” is a part of man that separates from the body at death and goes on living. But the Bible shows that Man IS a soul:

Quoting Gen. 2:7, 1 Cor. 15:45 says: "It is even so written: `The first man Adam BECAME a living soul.'" (This shows that the Christian Greek Scriptures agree with the Hebrew Scriptures as to what the soul is.)

With this in mind, the Bible says that souls in fact, die:

Ezek. 18:4: "Look! All the souls—to me they belong. As the soul of the father so likewise the soul of the son—to me they belong. The soul that is sinning—it itself will die." (Some translations say "the man" or "the person.")

Matt. 10:28: "Do not become fearful of those who kill the body but cannot kill the soul [or, "life"]; but rather be in fear of him that can destroy both soul and body in Gehenna."

Acts 3:23: "Indeed, any soul that does not listen to that Prophet will be completely destroyed from among the people."

For more, see

Do You Have an Immortal Soul?

“Soul” and “Spirit”—What Do These Terms Really Mean?

What Happens After You Die?

A Closer Look at Some Myths About Death

What really is Hell?