1. Because Jesus was baptized that way.
Jesus was immersed. The Bible says that Jesus was baptized in the River Jordan (Mark 1:9, 10). The Greek word, "eis", meant in or into. When Jesus came out of the water, the word "ek" was used, which means out of. So Jesus went into the water and came up out of the water.

"As soon as Jesus was baptized, He went up out of the water."
Matt. 3:16

"At that time Jesus came from Nazareth in Galilee and was baptized by John in the Jordan. As Jesus was coming up out of the water, He saw heaven being torn open and the Spirit descending on Him like a dove."
Mark 1:9, 10

"We were therefore buried with Him through baptism into death..."
Romans 6:4a

2. baptism in the Bible was by immersion.
In New Testament times believers were immersed. The apostle Paul wrote that baptism is a burial (Romans 6:4), and that's why both Philip and the eunuch went down into the water and came up out of the water.

"...then both Philip and the man went down into the water and Philip baptized him. When they came up out of the water..." 
Acts 8:38-39

What does the word itself mean? The word Baptize means "To dip under water" The Greek work Baptizo means "To immerse or dip under water". It best symbolized a burial and resurrection!

"We were therefore buried with Him through baptism into death..."
Romans 6:4a

3. The founders of denominations agree:
For more than 1200 years of Christian history, immersion was universally accepted as the way a person was baptized. Even the great Reformers such as Martin Luther, John Calvin, John Wesley acknowledged that immersion was practiced in New Testament times.

Martin Luther... "I would have those who are to be baptized to be entirely immersed, as the work imports and the mystery signifies."

John Calvin... "The word ´baptize´ signifies to immerse. It is certain that immersion was the practice of the ancient church."

John Wesley... "Buried with Him, alludes to baptizing by immersion according to the custom of the first church."