Generally speaking, for any kind of "-ism", the suffix "-ist" produces the nominal form and "-istic" produces the adjectival form. But there are some "-ist" suffixes that are acceptable or even more commonly used as an adjective. Examples include fascist, humanist, racist, socialist, absurdist, etc.
Why do these adjectives not take the usual derived form, i.e. fascistic, humanistic, racistic, socialistic, absurdistic?