The answer is simple: items in WoW are statistic-centric, and just about every character statistic helps Hunters. Other classes tend to be more heavily fixated on a few stats and don't need items that boost the rest, but Hunters benefit from almost every stat boost. Lets take a closer look:
There are five attribute statistics in WoW: intellect, spirit, strength, agility, and stamina. Of those five, strength and maybe spirit are the weakest attributes for Hunters.
Agility is the chief stat for Hunters, boosting ranged attack power by two for every point of agility, and adding to melee attack power, dodge, and critical strike chance. This puts Hunters into fierce competition with rogues for items that boost agility.
Intellect may be the second most important stat for Hunters. Intellect boosts the Hunter's mana pool, which is what he uses for special attacks. Hunters rely on special attacks to boost their damage output. This puts Hunters into competition with Mages, Priests, Warlocks, Shamans, and even Druids for items that boost intellect.
Stamina boosts hit points and so is important to every class. Mages often are resigned to never having enough hit points so many eschew stamina gear in favor of gear that boosts their mana pool and spell crit chance, but every class likes stamina gear.
Spirit is the least popular stat among WoW players. It increases the hit point and mana pool regeneration rate, and most players don't care that much about faster regen, preferring to rely on food and drinks after fights to replenish themselves. The exception will be in raid instances where the fights last a long time and players are locked into combat and can't eat or drink. Then spirit becomes more important. Interestingly, Hunters with a lot of spirit probably benefit more than other classes because their mana pool is shallow to begin with, so anything that boosts mana regen is helpful to Hunters.
Strength is arguably the least popular stat for Hunters. It boosts melee attack power by one point per one point of strength, and Hunters do their best to avoid having to melee. Still, Hunters do benefit from more strength. They often have to finish off an opponent with their melee attacks.
Beyond the basic stats, Hunters are also keen for items that boost their critical strike chance and attack power. This puts them in competition with Rogues and Warriors primarily, but Druids will want these items too.
Then there are weapons. Hunters use melee weapons like trinkets most and look for melee weapons that lead to increased range attack damage. This can be particulary infuriating to other classes that rely heavily on melee weapons -- namely Warriors and Rogues. They see Hunters rolling on a Dal'Rend's weapon and their blood boils. The Hunter wants it for its stat boosts, but the Rogue and Warrior look at it as a weapon they will use constantly in combat. And Hunters can wield one-hand and two-hand swords and axes, daggers, polearms, and even staffs.
That's why Hunters roll on everything. They can use everything, or at least get more use out of a wider variety of items than any other class. I don't think Blizzard planned it that way, but it's certainly the way the game is set up presently.