When you reconstitiute HGH, you're not supposed to shake it, so what happens if it's not treated right at the post office and gets thrown around. Powder is much more stable.
From RedBaron's informative post:
HGH is somewhat fragile by nature, and it needs to be protected from light and heat. HGH should be stored between 36 and 46 degrees Fahrenheit at all times both before and after its reconstitution.