Exorcism does not infallibly lead to a person being freed of demonic possession.
Therefore, it couldn’t be a sacrament, which always instantly results in the proper sacramental graces being given by God, so long as a person is properly disposed.
Example:
So long as I put no impediments before God, when I show myself to the priest in Confession, and when he absolves me, my sins are, in fact, absolved. The grace of the Sacrament is infused into my soul. I need not go back six or seven or eighty or a thousand times in order for absolution to take.
But exorcism can take a great deal of time, and it can require that the rite be prayed multiple times. Sometimes, it is God’s will that a person be freed soon; and other times, God permits possession to go on for weeks or months or years.