Most mammalian vocal folds taper to a triangular edge. In lions and tigers the fold has a thick, flattened profile, built from a deep layer rich in fat and elastin fibres. That squared edge deforms more evenly under airflow, so the tissue can be driven into large, low-frequency oscillations without an extreme rise in lung pressure.
Laboratory work on excised larynges has shown that the folds will self-oscillate under airflow alone, with no nervous input, producing the low fundamental of a roar. The other half of the system is the larynx's suspension: in roaring cats an elongated, partly ligamentous connection replaces the fully ossified hyoid chain, which lengthens the vocal tract and lowers the resonances.
The common shorthand that a lion roars "because its hyoid is not fully bony" is only part of the story, and it is not a clean dividing line. The snow leopard also has an incompletely ossified hyoid yet does not produce a true roar, which points back to vocal-fold structure as the more decisive difference.