Learn English – Why did -ful prevail instead of -full for adjectives

When you switch to the World English version of Oxford Dictionaries Online, their definition of usable has this little "spelling help:"

Usable can also be spelled useable, with an e in the middle: both are correct.

The US version simply lists useable as an acceptable variant of usable, and omits the side note.

Etymonline shows usable as being derived from the Old French usable. It further notes that the word was "not common before c.1840," so it would seem that useable is not an "archaic form" that fell into disfavor.

I would guess that useable came to be an acceptable variant because of standard usage. I would note, however, that some dictionaries do not list useable at all.

Basically the description given is the description used. Since English has so many words that come from Latin (either straight or aged in Norman French), it has tended to borrow whole sets of words formed on individual roots.

Thus, denominal "adjectives" in -al/ar were already formed in Classical Latin. Of course, classical grammarians would simply have called them Nouns -- "Adjective" is a modern Part of Speech.

There is no special term for these words, which all have their own peculiarities. Don't forget, every word in the language has its own individual history, the result of its millions of daily uses, in the mouths of millions of speakers, over thousands of years.

History is not something that happens to classes of words as a group, but to each word in its own way. Classification and generalization comes later; nobody noticed the Great Vowel Shift while it was going on, after all.