In much the world, vegetarianism is unheard of. Vegetarianism is largely a peculiarity of India and of the West.
According to Wikipedia, around 20-40% of Indians are vegetarian, mostly Jains and Brahmins. (Muslims are not, nor are people on the coasts, presumably thanks to fish supplies.) The reason is religious, meat being considered impure.
In the West, I would guess that about 2% to 5% are full vegetarians in most countries. (Wikipedia gives UK 3%, Canada 4%, USA 2.5-3%, Switzerland 2.3-5%, Netherlands 4.5%, Portugal 0.3%.) Maybe another 5% are semi-vegetarians. That means a total of around 10% are either full or semi-vegetarian. Women tend more to vegetarianism than men.
So, why is vegetarianism more common in the West. I assume it is a recent phenomenon linked to greater concerns over health and more altruism towards animals.