Because of the meaning of the word baptize. Baptize is actually a Greek word which was transliterated into English. The translation of baptize is actually “to immerse.”
Because of the symbolism of the act (Romans 6:4-5). Baptism is an act which symbolizes the believers acceptance of the death, burial, and resurrection of Christ as provision for the washing away of sin.
Because of the practice of the early church, which was obviously immersion (see such passages as Matt 3:6, 16; Mark 1:10; John 3:23; Acts 8:38-39, etc. The clearest picture of NT practice is found in Acts 8:26-40).
Because there was a perfectly good Greek word for “sprinkling,” and it is not used here.