Part of the reason Lethality is so much stronger than crit right now
Most Crit items are just raw stats that modify auto attacks. All the Lethality items have some kind of passive or active that grants additional effects along side the stats.
Only BorK and Merc Scimitar have actives and are usually built by "crit" adcs, and they're situational.
Zeal items do give some unique effects to autos, but it's still just a bundle of stats for their autos. I know that's what ADCs are supposed to be doing (auto a lot) but that's why casters who don't rely on autos that much for the bulk of their damage (Draven is a bit of an oddity)
Tl:Dr: Less stat sticks, More unique passives/actives for crit items.
Edit: Lethality items are also generally cheaper, especially since Dirk builds out of Longswords while BF Sword is a raw purchase.