Kevura, or burial, should take place as soon as possible after death. The Torah requires burial as soon as possible, even for executed criminals. This means that burial will usually take place on the same day as death, or, if not possible, the next day. Some Reform and other congregations delay burial to allow more time for far flung family to come to the funeral and participate in the other post burial rituals.
The traditional practice may have originated from the fact that Israel was, and is, a country with a hot climate. In Biblical times, there were few ways of keeping the dead body from decomposing. Not only would this be generally undesirable, but allowing the dead body of any person to decompose would be showing that person great disrespect. Decomposition would have occurred especially quickly in Israel due to the constant heat. Thus, the custom of burying the body as soon as possible. (Although the practice of embalming and mummification had advanced to a high level in Egypt, this, too, is considered disrespectful, since it involves a great deal of manipulation and the removal of bodily organs.) In addition, respect for the dead can be seen from many examples in the Torah and Tanakh. For example, one of the last events in the Torah is the death of Moses when God himself buries him: "[God] buried him in the depression in the land of Moab, opposite Beth Peor. No man knows the place that he was buried, even to this day." (Deuteronomy 34:6)