Because repentance is necessary to enter into the Kingdom of God. Without genuine repentance the church will produce lukewarm worldly so called Christians by name only.
Conversion comprises two elements, and that is repentance and faith.
Repentance is connected with sanctification and faith is connected with justification. Repentance and faith are like two sides of the same coin. Repentance is turning away from sin and faith is turning to Christ for salvation. The Greek word for repentance is metanoia This word comes with two words, meta and noia. Meta means to change as in metamorphosis, and noia means mind as in paranoia.