When you have firsthand knowledge of the faithfulness of Jesus, telling those around you (witnessing) is the natural, inevitable result.

When we have experienced the faithfulness and effectiveness of something we have faith in it/them and very openly recruit people to that brand, location or philosophy

Why is this not always the case with Christ?

The problem with most "faith" today is that it often instead of basing it on experiencing the faithfulness and effectiveness of the Gospel of Jesus through the Power of the Holy Spirit it is based on "false knowledge" (fear, control, pride and self-centeredness)