That is a good question, you have to consider that Christianity has numerous denominations. Protestants and Catholics make up the bulk of this faith. There are over a billion of each group, Catholics and Protestants, which is over 2 billion people. There is no other single religion with those numbers.
Why is it the largest for so long a period? There is truth and actual manifestations within the faith that cannot be denied, the realities of the faith. If people did not have these manifestations in their lifeline a personal one on one account, it could not last, a concept or philosophy cannot endure the test of time for thousands of years…..