It has nothing to do with Jell-O and everything to do with gelatin, aka powdered bone slime. Gelatin is a protein consisting of a triple-helix of amino acid chains. The amino acid chains separate when the gelatin is dissolved in hot water, and when the mixture is cooled, the chains link together again to form a semi-solid substance, in much the same way that epoxy resin hardens when the epoxide monomers link together to form polyepoxide plastic. That's not really a riddle, though.
This is incorrect, for as I've stated it has nothing to do with a product marketed as Jell-O
TM or any of its ingredients.
Then it's probably because "O" is a vocal and a lot easier to yell than say "K".