There are the obvious sanitary reasons why you would not want the carcass of a dead cow in a field that was being actively used for agriculture.
But let's dig deeper:
The dead cow is related to the red heifer in that it is a purification offering, the idea being that sin is a pollutant that requires a cleansing.
With the red heifer (Numbers 19), if a person touches a dead body, they have a priest mix the ashes of the red heifer with running water, and run that over the person who is cleansed.
The ritual must be done on day three and on day seven he is clean. But while unclean, they remain impure and must not enter the camp. That means they cannot participate in any of the community life, cannot offer sacrifices, cannot celebrate, and must be completely isolated and alone:
He is a leprous man, he is unclean: the priest shall pronounce him
utterly unclean; his plague is in his head.
And the leper in whom the plague is, his clothes shall be rent, and
his head bare, and he shall put a covering upon his upper lip, and
shall cry, Unclean, unclean. All the days wherein the plague shall
be in him he shall be defiled; he is unclean: he shall dwell alone;
without the camp shall his habitation be. Leviticus 13:44-36 KJV
After the seven days they are cleansed from touching the dead body and they are allowed to rejoin the community.
With the dead cow, it is the land that touches a dead body -- a murdered body -- and this pollutes the land (see how Abel's blood cried out from the ground). The land therefore needs purification as God resides in the land so the land is holy:
So ye shall not pollute the land wherein ye are: for blood it defileth
the land: and the land cannot be cleansed of the blood that is shed
therein, but by the blood of him that shed it. 34Defile not therefore
the land which ye shall inhabit, wherein I dwell: for I the LORD dwell
among the children of Israel. Numbers 35:33-34 KJV
As the person must be isolated from the rest of the community, the land must not be put to use during the time so that it does not interact with anyone in the community while it is being cleansed. The land is thus alone and "outside the camp". The person was isolated for 3 days before and 3 days after the washing, and was clean on the seventh day, so he was unclean during the days of labor and clean when the labor ceased. Similarly, the land must be cut off or unworked before and after the ritual and can only be clean if it rests.
that ye may put difference between holy and unholy, and between
unclean and clean. Lev 10:10 KJV
As the land is symbol for us, whom God inhabits -- e.g. Adam is the ground -- therefore the process of purifying the land follows similar principles to the process of purifying a person.
