As to why, to actually to be 100% sure you'd have to go yell at GameFreak. The rest of the world knows the same amount about this as you probably, and google doesn't want to give me any gems of information D:
However, you could probably still pick up some differences between the weathers.
Personally speaking, I'd say Rain and Sun were both more powerful weathers than Hail and Sandstorm ,effect-wise.
Rain and Sun both buff the power of moves of a type, decease the power of moves of a certain type, limit or increase the power of certain healing moves, have a variety of abilities directly related to them ( 6 for sun, 5 for rain), make certain moves less viable, and make other moves more viable
Sand and Hail damage certain Pokemon, affect certain healing moves, both affect Weather Ball like rain and sun, and hail affects Blizzard. They both have a few abilities directly related to them, but I'm pretty sure less than Rain/Sun with Hail having 3 and Sand 3. Sandstorm also buffs certain types Pokemon. That doesn't include the weather-summoning abilities tho.
Anyway, it's just personal, but I'd say the effects of Rain/Sun are probably stronger than that of Hail/Sand.
Also, Sunny Day and Rain Dance have more distribution then Sandstorm and Hail, so maybe the reduced the pp to try even it out a bit or something.