Why was eating meat on Fridays forbidden in the Catholic Church for so many years? I understand that the Catholic Church forbade people from eating meat on Fridays for many years.  It seems that it is no longer every Friday, but just during Lent.  Also, it is not all meat, but all meat that isn't fish (from what I understand).

How did this become considered a sin?  Why meat?  Why Fridays?  Why did it end?
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.