The religious situation in China is very complex and confusing because it's relatively unorganised, everyone uses different definitions and methods of breaking down the demographics. What the graph above calls 'Chinese Universalism' most people call D/Taoism or Chinese folk religion (in fact, this is the first I've heard of the term 'Chinese Universalism' and I know a thing or two about Chinese culture). Today Confucianism (along with Buddhism and some other elements) is really mixed into Taoism rather than something that stands on its own (which I suspect is what 'universalism' refers to). So by that definition Firaxis just went with by far the 6 largest religions in the world today plus Judaism (but the reasons for the latter will be fairly obvious to most).
These (except Judaism) are also the only religions today that have more than a few million followers, so it makes sense to draw the line there. If you include Sikhism it's hard to argue a whole bunch of other religions of similar size like Zoroastrianism, Bahai, Jainism, Shinto, Juche, Voodoo, Spiritism and various New Age-ey beliefs shouldn't also be included, not to mention some non-modern ones (e.g. the Roman state religion had a much wider following than Sikhism ever did, especially as % of the world population). Sikhism may by common definitions be among the largest of that group, but the differences are very small (on a population of almost 7 billion), the definitions very vague and this represents only a snapshot in time.
You could argue until the end of the world about which religions are most 'deserving' of a place in Civ without ever agreeing on anything, especially since this is such a sensitive subject for which there often aren't any very precise or widely agreed-upon definitions. It's also a very culturally-dependent thing: many Christians want to see Christianity split up in different denominations, Asians or people with an Asian background want to see a different distribution of Asian religions, history-minded people want to see some historic religions, etc.
But today (and indeed for most of history) the world is roughly divided in 3 major groups of organised religions: Abrahamic, Indian and Far Asian (yes, that's a gross oversimplification but so is everything else in Civ). Each of these is represented in the game with (by far) its two largest individual religions (though again, the Far Asian situation is quite messy). On top of that you have Judaism, which is a bit of a special case anyway. Without making things really complicated that's as good as Firaxis could've done, really. That, and make the game as moddable as they did
An elite Nihang unit would fit the bill.
