Perhaps the better question is, why not baptism? For the Christian, baptism is an outward sign of an inward grace. It symbolizes the saving work of God in one’s life. Jesus commanded His disciples to make other disciples, with part of this being baptism in the name of the Father, the Son and the Holy Spirit (Matthew 28:18-20).
The earliest Christians would never have imagined a person converting to Christianity without being baptized, but at the same time they do not tell us that God requires baptism in order to having a saving relationship with Jesus. It just makes sense for a person to declare that he or she has decided to follow Jesus as His disciple in baptism. Acts 8:12 says, “When they believed Philip as he proclaimed the good news of the kingdom of God and the name of Jesus Christ, they were baptized, both men and women” (NIV). This verse suggests an immediate baptism following belief. You may have waited months or even years after becoming a Christian to choose to be baptized. Whatever the timeline, if you're reading this and considering baptism, we encourage you to take this next step.