As has already been pointed out Alpha Acid stability is better in pellets/plugs than flowers. Why do producers make pellets out of flowers, like all things commercial, if in doubt, tick cost. This is how industry prefers to buy them obviously there are cost issues re transport, storage/mass etc.
Some info from:
http://homebrewandbeer.com/ingredients.html
Hops are available to the homebrewer in three forms:
Leaf hops, or hop cones, are whole hop flowers that have been picked, dried and compressed into bales. Because the processing for whole hops is minimal, most of the flowers' delicate glands containing the aromatic oils and resins are intact. However, because the surface of the hop flower, is exposed to air, they are more prone to going off and their oils breaking down. Therefore, freshness is essential when buying and using hop cones. If your recipe calls for hop pellets and you use cones, use about 25 per cent more cones.
Plug hops are whole hop flowers compressed into large pellets. The processing leaves the hop flower somewhat more intact than with hop pellets.
Hop pellets are pulverised hop flowers compressed into small cylindrical pellets. Many feel these are the best form of hop because there is minimum exposure to the air and the oils and aromas remain trapped inside the pellets. Usually, the pellets are sealed in a foil bag, which helps preserve them. If a recipe calls for hop flowers or cones and you use pellets, use about 20 per cent less pellets.
Hope this helps FG,
Screwy