If Heavenly Father is no respecter of persons, why does the bishop get served sacrament first before the rest of the congregation?
As the president of the Aaronic Priesthood in his ward, the bishop has been given the responsibility to oversee the administration of the sacrament. For example, if the priest officiating at the sacrament misstates the sacramental prayer and does not catch and correct his error, it is the bishop’s duty to correct him. (Note that when a stake presidency member or General Authority sits in sacrament meeting, it is he, not the bishop, who presides and assumes control over the meeting, including the sacrament.)
So when the administration of the sacrament takes place, it is reasonable that the bishop (or other higher authority with responsibility over the meeting) be served first, so that he can either approve the administration or instruct that it be redone. While this is not the only way such an end could be reached, it is the one common in the Church today.