Christians say, that the soil thrown on the coffin symbolizes that the body is transitory, but the main meaning of this act is binding the dead to the grave, so he/she will not escape. The same thing is doing symbolically with the living ones to curse them or with supernatural or "immortal" beings or so-called demons.
Unlike a fire, the earth decomposes the dead bodies slowly, so parts of the deceased are staying here for a while and may be misused by others in their favor.
Cremation is the fastest and safest for leaving the human realm and continuing his/ her journey, which was used to practice worldwide a long time ago. Graves and burial chambers are opposite to burning ghats in their actions. Nowadays, burning bodies in the fire is not really popular as a funeral ceremony for any reasons, from religious to economic. Maybe, indifference and ignorance is the main cause of Earth pollution with dead bodies. Maybe selfishness or different values and a great fear of Life & Death are the reasons why people want to keep their embalmed "beautiful" bodies in the earth
Besides, most of the modern funeral procedures are no more than meaningless technological actions with no knowledge and idea behind them, except greediness, fear, or laziness, when relatives don't care and don't know what to do with the deceased ashes or body.
We don't think that there is only one "true" set of rules on how to be buried, how to die, or how to live properly. We respect a person's "free will" and his own decisions, but why not be grateful and respectful in turn to Nature?