Because the host is the Body, Blood, Soul and Divinity of Christ under the guise of bread and wine.
We are to abstain from meat. Not bread or wine. Because meat, in ancient times and in 3rd world countries, were only consumed during celebrations (too expensive otherwise). Friday is a day of remembering and mourning, not of celebration. Bread and wine were commonly eaten every day.
HOWEVER, since the mass is both a sacrifice and a celebration, do we need to reflect only on the sacraficial aspects and not the celebratory (Lamb’s supper etc) aspects?
In the 2,000 years of fasting, everyone has understood that we fast animal flesh. Fish is meat but, Latin Catholics eat fish on Fridays during Lent. We know that “fasting meat” does not include the Eucharist.
The Body and Blood of Christ have the outward appearance of bread and wine. Also the Church defines the penitential practice and it does not usually include the Eucharist, however it does if one has not met the time period for the Eucharistic fast. Also in the eastern Catholic churches, there are days during lent where the Eucharist is not available for reception, and this is a penitential practice.
There are two type of penitential practice involving eating in the Latin Church:
*]abstinence: restricting the choice of foods
*]fasting: restricting the quantity of food
Latin Canon Law:Can. 1250: The penitential days and times in the universal Church are every Friday of the whole year and the season of Lent.
Can. 1251: Abstinence from meat, or from some other food as determined by the Episcopal Conference, is to be observed on all Fridays, unless a solemnity should fall on a Friday. Abstinence and fasting are to be observed on Ash Wednesday and Good Friday.
Can. 1252: The law of abstinence binds those who have completed their fourteenth year. The law of fasting binds those who have attained their majority, until the beginning of their sixtieth year. Pastors of souls and parents are to ensure that even those who by reason of their age are not bound by the law of fasting and abstinence, are taught the true meaning of penance.
§1. A person who is to receive the Most Holy Eucharist is to abstain for at least one hour before holy communion from any food and drink, except for only water and medicine.
§2. A priest who celebrates the Most Holy Eucharist two or three times on the same day can take something before the second or third celebration even if there is less than one hour between them.
§3. The elderly, the infirm, and those who care for them can receive the Most Holy Eucharist even if they have eaten something within the preceding hour.
The eastern Catholic canons are different and the practice during a strict fast also prohibits dairy products.