Why do supercells make 5-cm hail when ordinary storms usually cannot?

Answer: Sustained storm updrafts

Lower cloud ceilings — Not quite. A low cloud base can matter for some storm hazards, but it is not what makes hail huge. The hail-growth problem is mostly aloft: can the storm hold the stone near supercooled water long enough? A short-lived storm may produce plenty of ice but not give it time to become large.

Colder air near ground — No. Colder air near the ground can help hail survive its fall, but it does not explain giant growth inside the storm. The key work happens aloft, where stones need time near supercooled water and strong upward winds.

Sustained storm updrafts ✓ — Correct. Big hail needs time inside a storm, plus upward wind strong enough to hold a heavy ice ball aloft. NSSL says supercells have sustained updrafts, and hail about 5 cm or larger is generally linked with them. Ordinary multi-cell storms can make many stones, but their mature stage is often too short.