capitalization - Why is quixotic not Quixotic (a proper adjective)?

Adjectives derived from proper nouns are known as proper adjectives, and are capitalized:

A piece of writing could be Shakespearean, not shakespearean.
A person may be Canadian, not canadian.

Even Chrome's spellchecker sees these as correct and incorrect.

However, quixotic is written in lower case, despite coming from the name of the character Don Quixote. Similarly, draconian laws are named for Draco, a particularly brutal senator from ancient Athens.

Does anyone know the reasoning behind some proper adjectives not being capitalized in common usage?

Answer

In a comment posted years ago to the question Why is "biblical" the only proper adjective to not use upper case? I listed some other exceptions to the general rule that the first letter of an adjective derived from a proper name is normally capitalized. Only the letters q, w, x, and y did not yield an example (for some reason, I failed to notice quixotic):

arabesque, byzantine, caesarean, draconian, epicurean, faradic, galvanic, herculean, italic, jesuitical, kabbalistic, lilliputian, mercurial, nazi, oedipal, pyrrhic, rubenesque, spartan, terpsichorean, utopian, voltaic, and zephyrous

Why do these exceptions occur? The not-very-satisfactory answer seems to be that common usage determines whether an adjective based on a proper name is initial-capped or lowercased. Dictionaries provide their spelling preferences based on what amount to the found objects of preponderant real-world usage in each case; and thenceforth, real-world usage (to the extent that it is influenced by people who look things up in dictionaries) tends to reflect the dictionary treatment. The very circularity of the process makes it extremely difficult to determine where and when the critical decision regarding initial cap versus all lowercase got made.

I don't see how else to explain why Oedipal is usually initial-capped when it refers to the mythical character Oedipus (as in "Oedipal resistance to fate") but usually lowercased when it refers to Freud's Oedipus complex (as in "oedipal feelings"), although the complex is explicitly named after Oedipus.

Bryan Garner, Garner's Modern American Usage, second edition (2003) concludes that trying to explain exceptions to the normal rules about what gets capitalized and what doesn't is a fool's game:

There is simply no way to reason out why Stone Age is capitalized but space age is usually not, why October is capitalized but autumn is not, why in scientific names the genus is capitalized but the species is not—even when the species name is derived from a proper name {Rhinolophus philippinensis}.

Ultimately, capitalization conventions rest on strong general tendencies tempered by exceptions that are neither consistent nor explicable.