The Adherents.com "Major Religions" list presented on this web page differs from classical lists because it draws more from an extremely large body of contemporary affiliation data, rather than relying heavily on the lists and texts of past commentators (Hudson Smith, Noss, Barrett, etc.).
There are many distinct religions or religious movements which have more adherents than some of the classical world religions, but which are not part of the classical list for various reasons. These reasons include:
the religions which are not included on the classical list are too new (Scientology, Neo-Paganism)
they are concentrated in only one country (Cao Dai, Ch'ondogyo, Tenrikyo)
they lack identifiable central organizations or unifying scriptural literature (Neo-Paganism, New Age, Spiritism)
their adherents primarily name a different, more established traditional religion as their religious preference (most practitioners of Vodoun are nominal Catholics, practitioners of New Age religions are often nominally Protestant, Catholic or Jewish)
their religion is still strongly associated with a major religion from which it arose, but no longer wishes to be an official part of (Tenrikyo and many other Japanese New Religious Movements, as well as many religions emerging from Indian/Hindu environments)