Hail has long been considered an underperforming weather condition, and further belonged to a type that was already pretty horrible defensively (Ice). So it was reworked to become Snow. In fact, this was done as a bit of a trial run in Legends: Arceus.
Instead of Hail's turn-on-turn damage to non-Ice types, Snow now increases the Defense of Ice type Pokemon by 50%. This will also stack with Aurora Veil and Reflect, resulting in Ice type Pokemon taking very little damage from physical attacks. It also retains the Blizzard accuracy bonus that Hail had, and Abilities that were triggered by Hail in the past (Slush Rush, etc) are now triggered by Snow instead.
Additionally, there were a handful of strategies that were difficult to make use of with Hail on account of the turn-on-turn damage, but now that it is no longer present in Snow, the non-Ice types required for those strategies have a lot longer expected lifetime.
