Blame families... and marketing executives
One theory is that Mormons indulge in the gelatin simply because they have to abstain from alcohol, coffee, and tea. JELL-O and its jiggly allure is one of the only acceptable vices for rebellious LDS teens.
In reality, JELL-O was traditionally aimed at families. And that has more to do with the connection than any scripture.
According to Christy Spackman's piece on Mormon cuisine stereotypes (specifically JELL-0), sales fell dramatically in the late 1970s. Desserts became more elaborate, structured affairs -- far from the quick and simple template that JELL-O was founded on.
The company re-branded itself as a way to bring family members together with marketing campaigns featuring America's Dad, Bill Cosby (hindsight: yikes). As Mormons tend to have larger families -- their contraception limitations make the Catholic-Irish look like a Nelly video -- this was a natural fit, and a big opportunity for the flagging brand. Marketing was intensified in Utah, and kids who were generally prohibited from more scandalous, sugar-packed treats ate the stuff up. Like, actually ate the stuff up. At record clips.