People commit all kinds of sins. The 1962 pope knew the "worst" sins committed by priests are not car stealing, bank robbery, burglary, or even murder. The sins he explicitly mentioned are perverted sex acts. And he wanted to keep those a BIG secret.

They created another fear, as a control mechanism.
Reveal that 'worst' secret, get thrown out of the priesthood, find yourself excommunicated, and your soul goes to hell, right?