Why are some "-ist" suffixed words used as the adjectival form over the more common "-istic"?

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?

The suffix -istic is generally equivalent to the suffixes -ist and -ic but in some cases it suggests a different connotation. Since every term has its own etymology and usage it is difficult to find a general rule.

• equivalent to a combination of -ist and -ic but in some words having a less specific or literal application and sometimes a mildly pejorative force, as compared with corresponding adjectives ending in -ist: communistic; impressionistic. (from TFD)

-istic:(Etymonline)

• adjectival suffix, from French -istique or directly from Latin -isticus, from Greek -istikos, which is adjective suffix -ikos (see -ic) added to noun suffix -istes (see -ist).