I think you just need time. I get two Spiders and two Gladiators from the item boxes. I thought I'll never get a Galaxus torso, but last week I got one. With the Apocalypse I have the same situation like Aeacus. But I think this:
"The reason why you don't get a Spider torso may be because not all legendary items are available for all." Is totally uncorrect, in time you'll get them.
If you read correctly then i said it "may be" so, not that it certainly is so.
It's just, i have bought over 100 item boxes and there are legendary items that i've never gotten.
Here are all legendary items in range of 28-30 that i've never gotten:
Galaxus Model B (lvl30)
Legs (lvl30) (+60 HP)
Iron Shredder (lvl28)
Shotgun X Mark 2 (lvl30)
Shotgun X Mark 3 (lvl30)
Apocalypse Mark 3 (lvl30)
Though, a developer or someone who knows exactly how the item boxes system works should tell how it really is.