Why do many churches use grape juice in communion? Some Baptists and many other evangelicals use grape juice in communion. Is this explained on any catechism (or systematic theology available online) ?

Does the reason for not using wine have to do with keeping oneself pure or the moral problems with alcohol?
The Protestant practice of traditionally substituting grape juice for wine during communion must largely be credited to one man - Thomas Bramwell Welch

From Wikipedia:


  While some Christians consider the use of wine from the grape as
  essential for the validity of the sacrament, many Protestants also
  allow (or require) pasteurized grape juice as a substitute. Wine was
  used in Eucharistic rites by all Protestant groups until an
  alternative arose in the late 19th century. Methodist dentist and
  prohibitionist Thomas Bramwell Welch applied new pasteurization
  techniques to stop the natural fermentation process of grape juice.
  Some Christians who were part of the growing temperance movement
  pressed for a switch from wine to grape juice, and the substitution
  spread quickly over much of the United States, as well as to other
  countries to a lesser degree. There remains an ongoing debate between
  some American Protestant denominations as to whether wine can and
  should be used for the Eucharist or allowed as an ordinary beverage,
  with Catholics and some mainline Protestants allowing wine drinking in
  moderation, and some conservative Protestant groups opposing
  consumption of alcohol altogether. (emphasis added)


Anyone with basic knowledge of wine making knows that once grape juice is successfully squeezed and collected, it doesn't remain as juice for very long. Unpasteurized grape juice, if not consumed within a few days of harvest, will quickly ferment, rendering it not consumable (unless treated and stored to begin the wine making process). 

As the excerpt from Wikipedia above states, prior to the turn of the 20th century "grape juice communion" was practically non-existent. Before the modern discovery of the pasteurization process, it would have taken a significant amount of effort to have enough freshly squeezed juice for a congregation readily available every Sunday (or less frequently depending on the denomination). 



So, contrary to the traditional presuppositions held by Temperance Movement grandchildren, the theological "roots" of sipping grape juice instead of wine during communion has more to do with pasteurization and prohibition than it does biblical exegesis.