Why are two religions not ‘isms’

[Gustav]

While Neopaganism and Polytheistic Reconstructionism are both -isms, and some of the many religions within its umbrella are too, just as many aren't.
e.g.:
Druidry (though this is called Druidism just as often)
Wicca (though some of its subsets, i.e. Gardnerianism are -isms)
Asatru
Heathenry
Religio Romana
Romuva
Rodnovery

wicca>wiccan>wiccanism ?

 

[Gustav]

Not if it had always been called that tho...

true true but might there be some underlying linguistic convention?
i propose a move to linguistics

languages have "ish", "ic" and "ese" (common forms)

english
englic
englese

/puzzled

 

[Light Travelling]

Ok I have ism as meaning
“ a doctrine; a belief (or system of beliefs) accepted as authoritative by some group or school”

I think I am really asking is there a reason that western (colonial) language does not consider Islam and Christianity worthy of an “ism” under this definition?

Definitions of ism on the Web:

 

[Maika]

wiccaism.