Because

Joshua 6:17 KJV

And the city shall be accursed, even it, and all that are therein, to the LORD: only Rahab the harlot shall live, she and all that are with her in the house, because she hid the messengers that we sent.

The Bible says that Rahab was saved because she hid the spies. This should not be taken to mean that the act of hiding them saved her, but what is demonstrated and represented in the act. 

The act demonstrated faith

·         There was faith in the God of the Jews

·         There was faith in the word of the these men

·         There was faith in the protecting hand of God 

She believed that the God of the Jews was not only able to defeat the enemy but to save her alive in the midst of the storm. She believed that God would not only bring condemnation upon sinners but forgiveness upon repentance

The act demonstrated a change of heart

The cup of their iniquity was full and hers had contributed to that fill. Her lifestyle, her habits her manners had all been poured into the cup of Jericho's iniquity. But 

·         She had heard of the God of the Jews

·         She had believed the word concerning Him

·         She had accepted that condemnation was just

·         She had hoped in salvation and

·         She had then received it

Even though in this passage the salvation is a promise and not affected as of yet, God was making provision for her actual salvation. 

The act demonstrated a new kind of being

This was completely out of the character of one of her character. Her previous occupation proved that her natural inclination was to do what was convenient and easiest to make a buck. She had always taken the low road heretofore. This act represents a different direction in her life. She did the dangerous and difficult thing. She did the thing that was difficult immediately in anticipation of receiving that which was greater eventually.