Question:
Why is it forbiden in Islam to cremate the body of a dead one?

Response:
The body of the dead is given equal protection in the law of Islam as that of the living. This reverence of the dead stems from Islam's view of the special place of human beings in Allah's scheme as He says in the Qur'an
"We have conferred dignity on the children of Adam"
This dignity is expressed in the Sharia (Muslim Law) in according sanctity to the human person alive or dead. According to the law causing unauthorized damage to a dead human warrants the same penalty as if it were inflicted on a living human.
Interfering with the dead body is allowed by jurists only for medical research, organ donation or post-mortem carried out to ascertain the cause of death. Cremation may be allowed if it is the only way of combating an epidemic.
In short you may interfere with the dead only if it helps the living. At present cremation serves no particular interest for the living