Vegetarianism started in ancient India and Greece. Now it is very widespread.
I wonder what was their reason for abstaining from meat  at that time
They didn't want to kill any animals and it was supported by religious groups.
thats great, it means it started out as a social cause
History of vegetarianism
The earliest records of vegetarianism as a concept and practice amongst a significant number of people are from ancient India and the ancient Greek civilizations in southern Italy and Greece. In both instances, the diet was closely connected with the idea of nonviolence toward animals (called "ahimsa" in India), and was promoted by religious groups and philosophers.
Following the Christianization of the Roman Empire in late antiquity (4th–6th centuries), vegetarianism nearly disappeared from Europe. Several orders of monks in medieval Europe restricted or
West but protected in much of India.
History.
Analyzing the history of vegetarianism and opposition to it from ancient Greece to the present day, literary scholar Renan Larue found certain commonalities in what he described as carnist arguments. According to him, carnists typically held that vegetarianism is a ludicrous idea unworthy of attention, that mankind is invested with dominion over animals by divine authority, and that abstaining from violence against animals would pose a threat to humans. He found that the views that farmed animals do not suffer, and