So if the majority is Atheist its a "niche"?
Or if the majority was some kind of Pasta Worshiping Occult its a niche?
I2 is a niche, it fits the spazzes that can deal with I2, and those who thrive in that.
A site for new born mothers with inverted nipples wanting to breastfeed is a niche for those women. Not all women with inverted nipples will feel at home there, but the place will be catering a specific group.
AFN caters different needs than I2 does. There is an overlap in members, but the overall tendency of the sites is that different that not all members of AFN will be able to feel at home here, and not all the members of I2 will be able to feel at home at AFN. (Would be quite disastrous for both if the memberships were exchanged)
A shop aimed at frilly lace caters a niche market. A shop selling expensive sound equipment caters a niche market too.
There is no judgement in the word niche. Only saying it is not a "for all" supermarket format.