"If the bread is consecrated to be the body, blood, soul and divinity of Christ, then why is there a need to consecrate the wine as well?"
Christ himself at the Last Supper took both the bread and the wine, blessed them and gave thanks (Luke 22:19-20), and since we are instructed to "do this in rememberance" of him, the priest who acts "in personae di Christo" (in the person of Christ).
Secondly, it's dictated by canon law:
THE RITES AND CEREMONIES OF THE EUCHARISTIC CELEBRATION
Can. 924 §1. The most holy eucharistic sacrifice must be offered with bread and with wine in which a little water must be mixed.
§2. The bread must be only wheat and recently made so that there is no danger of spoiling.
§3. The wine must be natural from the fruit of the vine and not spoiled.
Can. 925 Holy communion is to be given under the form of bread alone, or under both species according to the norm of the liturgical laws, or even under the form of wine alone in a case of necessity.
Can. 926 According to the ancient tradition of the Latin Church, the priest is to use unleavened bread in the eucharistic celebration whenever he offers it.
Can. 927 It is absolutely forbidden, even in extreme urgent necessity, to consecrate one matter without the other or even both outside the eucharistic celebration.
Can. 928 The eucharistic celebration is to be carried out in the Latin language or in another language provided that the liturgical texts have been legitimately approved.
Can. 929 In celebrating and administering the Eucharist, priests and deacons are to wear the sacred vestments prescribed by the rubrics.
Can. 930 §1. If an infirm or elderly priest is unable to stand, he can celebrate the eucharistic sacrifice while seated, but not before the people except with the permission of the local ordinary; the liturgical laws are to be observed.
§2. A blind or otherwise infirm priest licitly celebrates the eucharistic sacrifice by using any approved text of the Mass with the assistance, if needed, of another priest, deacon, or even a properly instructed lay person.
Note Canon 925, specifically: "... or even under the form of wine alone in a case of necessity". There is a member of the parish that I attend that is wheat intolerant and is extremely sensitive to wheat products. Out of necessity and as canon law dictates this person receives Communion when drinking the precious blood, because Christ is truly and really present - his body, blood, soul, and divinity - under both species of the Eucharist, in every particle of bread and in every drop of the wine.
So in short: Christ, from whom all authority the Catholic Church received, as he consecrated both the bread and the wine at the first Mass, so must our priests do so as Christ instructed.