They are that many because they were predominantly a proselytising religion. They had many crusades like some of the others and went to many foreign lands. Otherwise, they would have been much smaller. Generally they are about 27 - 33% and not usually more and some are not practising.
In England, I believe that the Home Office has a record of over 500 Christian sects. But they are far from being the only crusaders. India has 1.5 billion people and so does China and so population does have its say. Still you will know that Christianity went to India and Africa as well.
So they are so many because of proselytising and so little because they are not the worlds largest in terms of population and culture. Many Indians and Chinese have Buddhist, taoist, confusionist, zain and Hindu backgrounds, as well as those who practice Spirituality in its pure form known as Yoga Philosophy. (Meditation and strict adherence to Dharma (righteousness) but without the constriction of the rites, dogma's and other religious trappings.
We are generally influenced by our countries doctrines and that of those who invade our soil.