To answer the question that was posted (why do Mormons abstain from caffeinated drinks), rather than answer a question that was not (why are Mormons required to abstain from caffeinated drinks), here is my response. Some Mormons abstain from caffeinated drinks because they have extrapolated that since tea and coffee are prohibited and both contain caffeine, it must be the caffeine that is bad.

As has already been repeated several times, this is nothing but personal opinion and is not an official practise. It is simply personal preference and tradition.