Why are hail storms always brief?
Hail is created within a thunderstorm by updrafts that carry water droplets to an altitude where temperatures are lower than the freezing point for water (32°F or 0°C). This freezing process forms a hailstone.  This process can repeat itself each time increasing the size of the hailstone until the hailstones become too heavy to be picked up again.

The creation of hail requires high wind speeds that may only be sustained in the storm for a brief period of time.  Longer periods of hail are possible but typically require large strong storms such as tornadic supercell thunderstorms.

References:
   How hail is created.
   Size of Hail vs wind speed