The Catholic Perspective

To understand the reason you must first understand that the Catholic religion follows a very specific set of rules a guidelines referred to as "Canon Law."  This is one of the longest standing legal systems in the world and it is because of Canon Law that Catholics abstain from eating meat on Fridays.  Now, if you know anything about the Catholic religion, you may know that there are things called "Fish Fries" held on Fridays.  The reason they can eat fish on Friday is because Canon Law differentiates meat and fish.

This isn't because they wanted to make up confusing laws, but literally because of the words chosen.  The word, in the Latin translation of Canon Law 1251, used to describe meat is "Carne" which is literally defined as meat, but it doesn't include fish in that definition.  Fish was defined by a different word.  The text of Canon Law 1251 can be read here:

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. (link to text)

So, when the above Canon Law states "Meat" it is specifically excluding fish because the latin word "Carne" doesn't include fish in the definition.  This is why Catholics view eating fish on Fridays as being acceptable, and because of the misunderstanding about the translation, people erroneously view that act as hypocritical.  Using different definitions, including words based off of more specific translations, has lead to a lot of confusion in this aspect.