Objection 1. It would seem that the matter of this sacrament need not be consecrated by a bishop. Because the consecration of the Eucharistic elements surpasses that of the matter in this sacrament. But a priest can consecrate the matter in the Eucharist. Therefore he can do so in this sacrament also.
Objection 2. Further, in material works the higher art never prepares the matter for the lower, because the art which applies the matter is more excellent than that which prepares it, as stated in Phys. ii, text. 25. Now a bishop is above a priest. Therefore he does not prepare the matter of a sacrament which is applied by a priest. But a priest dispenses this sacrament, as we shall state further on (31). Therefore the consecration of the matter does not belong to a bishop.
On the contrary, In other anointings also the matter is consecrated by a bishop. Therefore the same applies to this.
I answer that, The minister of a sacrament produces the effect, not by his own power, as though he were the principal agent, but by the efficacy of the sacrament which he dispenses. This efficacy comes, in the first place, from Christ, and from Him flows down to others in due order, viz. to the people through the medium of the ministers who dispense the sacraments, and to the lower ministers through the medium of the higher ministers who sanctify the matter. Wherefore, in all the sacraments which require a sanctified matter, the first consecration of the matter is performed by a bishop, and the application thereof sometimes by a priest, in order to show that the priest's power is derived from the bishop's, according to Psalm 132:2: "Like the precious ointment on the head," i.e. Christ, "that ran down upon the beard of Aaron" first, and then "to the skirt of his garment."
Reply to Objection 1. The sacrament of the Eucharist consists in the consecration of the matter and not in its use. Consequently, strictly speaking, that which is the matter of the sacrament is not a consecrated thing. Hence no consecration of the matter by a bishop is required beforehand: but the altar and such like things, even the priest himself, need to be consecrated, all of which can be done by none but a bishop: so that in this sacrament also, the priest's power is shown to be derived from the bishop's, as Dionysius observes (Eccl. Hier. iii). The reason why a priest can perform that consecration of matter which is a sacrament by itself, and not that which, as a sacramental, is directed to a sacrament consisting in something used by the faithful, is that in respect of Christ's true body no order is above the priesthood, whereas, in respect of Christ's mystic body the episcopate is above the priesthood, as we shall state further on (40, 4).
Reply to Objection 2. The sacramental matter is not one that is made into something else by him that uses it, as occurs in the mechanical arts: it is one, in virtue of which something is done, so that it partakes somewhat of the nature of an efficient cause, in so far as it is the instrument of a Divine operation. Hence the matter needs to acquire this virtue from a higher art or power, since among efficient causes, the more prior the cause the more perfect it is, whereas in material causes, the more prior the matter, the more imperfect it is.
Source: Summa Theologica (New Advent)