Why do incalculable, immovable and noticeable take A, while irresistible, incorrigible and negligible take I? It's the kind of stuff that makes new English learners throw their hands up in despair. This issue can drive even a grammar columnist nuts. I know because when I began to research -ables and -ibles, I found myself wading through a swamp of confusing and contradictory advice.
"Garner's Modern American Usage" calls -able a "living suffix," but says that "-ible is no longer a combining form in English."
A bona-fide suffix can combine with another word to create a brand-new compound that isn't in the dictionary but is nonetheless legitimate. A clappable performance. A spoonborne virus. A cubiclewide policy. A ferretlike man. You probably won't find these in your dictionary or your spellchecker, but they're legitimate because they follow a legitimate formula: Real word plus real suffix equals legitimate word.
So because -able is a real suffix, you can attach it to just about any verb to make your own adjective. Kissable, discardable, walkable, writable, knowable, keepable — the rules governing suffixes make these all acceptable.
But if, as Garner says, -ible is no longer a real suffix combinable with other words, then it's not like -able. You can't tack in on to verbs of your choice to create words like kissible, discardible, walkible, knowible, writible and keepible.
Then what, you ask, is -ible doing in words like legible, collectible, edible, visible, permissible, credible, compatible and digestible? Those aren't compounds you assemble yourself. They're bona-fide words, sanctioned by the dictionary. And if the dictionary spells it with an -ible, that's the way to go.