From the USSCB:
19. Changing circumstances, including economic, dietary, and social elements, have made some of our people feel that the renunciation of
the eating of meat is not always and for everyone the most effective
means of practicing penance. Meat was once an exceptional form of
food; now it is commonplace.
20. Accordingly, since the spirit of penance primarily suggests that we discipline ourselves in that which we enjoy most, to many in our
day abstinence from meat no longer implies penance, while renunciation
of other things would be more penitential.
I think the implication here is that meat, with the exception of fish, which was commonplace, was at the time of the Church Fathers an exceptional or "celebratory" form of food. Since we, at least in America, treat meat as more of a staple food, it's no longer so strongly associated with celebrations or feasts. That said, abstaining from it is still a worthy form of penance, is still encouraged, and is still required on Fridays during Lent.