Why is the coldest storm top not the best place for hail to grow?

Answer: Liquid water runs out

Ice becomes too soft — No. Ice does not become soft because the cloud top is extremely cold. The issue is supply, not texture. A hailstone in air that is too cold and depleted of liquid droplets can be preserved, but it will not gain much mass.

Liquid water runs out ✓ — Correct. Hail does not grow just by being cold; it grows by collecting supercooled liquid droplets that freeze onto it. NSSL notes that at very high altitudes, below about -40 F, liquid water has already frozen, and hailstones need liquid water to grow large. The sweet spot is cold enough to freeze new layers but wet enough to feed them.

Sunlight breaks hail — No. Sunlight is not the growth limiter inside a towering storm cloud. The useful surprise is that maximum cold is not maximum growth. Like a freezer with no groceries, an ultra-cold cloud top lacks the liquid droplets needed to build the stone.