It is recommended because it is the easiest way to fufill the penitential Friday obligation of canon 1251.
USCCB Complementary Norm: Norms II and IV of
Paenitemini (February 17, 1966) are almost identical to the canons cited. The November 18, 1966 norms of the National Conference of Catholic Bishops on penitential observance for the Liturgical Year continue in force since they are law and are not contrary to the code (canon 6).
From Nov 18, 1966:
“3. Among the works of voluntary self-denial and personal penance which we especially
commend to our people for the future observance of Friday, even though we hereby
terminate the traditional law of abstinence as binding under pain of sin, as the sole prescribed means of observing Friday, we give first place to abstinence from flesh meat. We do so in the hope that the Catholic community will ordinarily continue to abstain from meat by free choice as formerly we did in obedience to Church law.”
nccbuscc.org/norms/norms.htm
Canon 1251 – Abstinence from eating meat or another food according to the prescriptions of the conference of bishops is to be observed on Fridays throughout the year unless they are solemnities; abstinence and fast are to be observed on Ash Wednesday and on the Friday of the Passion and Death of Our Lord Jesus Christ.
Canon 1253 – It is for the conference of bishops to determine more precisely the observance of fast and abstinence and to
substitute in whole or in part for fast and abstinence
other forms of penance, especially works of charity and exercises of piety.
trosch.org/for/the/abs-clws.htm