I just want to bring up that several ADCs can rush a Shiv first, some can build BotRK first, some can build BT first, some can build Ghostblade first, and some even build hurricane first.
But in the end, it all ends up being the same thing. IE/trinity force, BT/BotRK, shiv/ghostblade/pd, boots, LW, banshees/merc scimitar/second lifesteal item. It's kind of the same case other champion roles as well though (AP casters go pretty much the same build minus the mana regen item being tear vs. chalice or just double dorans), but the difference is that the varying builds can all be strong. For ADCs, unless you are specifically Draven or Kalista who are strong irregardless of items, IE is going to be stronger 90% of the time based on luck-RNG. Even Vayne who synergizes with BotRK can't do much.
There's also more flexibility for building in other roles. If you're behind vs. an AD Caster, you can build Seeker's Armguard and perhaps Dorans to survive laning phase. If you're losing vs. a Riven you can just rush Frozen Heart and out-wet noodle her. For ADCs, if you're behind, you have no choice but to rush the item they're rushing as fast as you can, from behind. Enemy ADC has BF sword, Pickaxe, and Cloak whereas you only have Pickaxe and Cloak? Nothing you can do to mitigate this disadvantage unless they massively outplay or your jungler ganks. You can't deviate from your build path at all or else you risk being even more behind in the mid-game. You just have to catch up to their IE as fast as you can, which they destroy your chances of doing even more.
I didn't really type this out clearly (too lazy to explain my thoughts properly), so TL;DR:
TL;DR: IE is oppressive because it is luck based, build path and timing is severely limited for ADCs compared to other roles. You CANNOT deviate from standard cookie-cutter build.