Why? They developed a strong temperance tradition over the years, in large measure because they saw alcohol as a major destructive force in society. Many of these Protestant groups also have condemned tobacco for the same reason. This was especially true such denominations as the Baptists and Methodists.
I was watching some TV program yesterday which said that 1700 college students die each year in the USA because of alcohol-related causes - maybe binge drinking, drunken driving, other accidents resulting from alcohol, alcohol-escalated arguments that lead to fatal fights, etc.
There are so many negative effects, everything from broken marriages to lost jobs to severe poverty to premature deaths through accidents or diseases..
The concept is that even a thimbleful of wine can throw an alcoholic off the wagon - so to speak. So, the simplest and safest policy is to abstain from all alcohol and serve grapejuice at communion. By the way, it has the same positive effects as red wine.
Since Protestants do not believe in transubstantiation, the elements are not viewed as so essential. This applies equally to the bread used as well as the 'wine' served. Many Protestants consider grapejuice unfermented wine - the unfermented 'fruit of the vine' - and some would argue that much wine in the time of Jesus (even most) probably was unfermented - consumed before it had time to ferment.
Take John Wesley, for example. He looked around and saw what enormous damage alcohol was doing in England, especially among the poor, who could least afford it. In many cases they drank excessively to escape their problems, making those problems even worse. Whenever and wherever Wesley preached the pub owners condemned him. One of the General Rules he passed along to early Methodists was abstaining from alcohol beverages. To him it was a pressing social issue to be faced, just as he condemned slavery and other societal evils.
One other thing. When people eat out, alcohol always adds enormously to their bill. A $10-15 meal is one thing. Add even a glass of wine and that bill goes up another $5-7. So, one can look at the matter from the economic point of view as well.
No problem with social drinking, if it is just that. Sadly, millions start out that way and end up as addicts. For millions alcohol - yes, even wine - becomes a drug, little different in its impact on some than heroin or cocaine. Winos are alcoholics, too. The safest course is to abstain.
On the other hand, thank God we don't have Prohibition. However, I am sympathetic to teetollers. Did I spell that right? I don't think so, Then, of course, the Mormons condemn tea, coffee and other hot drinks ala Joseph Smith. But Mormons are not Protestants.
God bless all people, of whatever faith, who seek to live lives that reflect the love commanded and commended to us by Christ. That is the core of our Christian faith, not the tribal trivia that divides us.