Conqueror only lasts for 3 seconds on marksmen before going on CD. This is not conducive to sustained fights. They then have to remain in combat another 3 seconds in order to restock this keystone back up; the teamfight literally has to last 6 seconds for them to get more than one use out of the keystone, where as melee champions can stack it up and it remains up indefinitely as long as they stay in combat.
Between conquerors lengthy CD and decently long time to stack up, it's only good in lane on ADCs who are guaranteed to have lane dominance, IE Draven or MF. Tristana's short range can lead her to being bullied out meaning there is a lot of down time for the keystone. Jhin has better options in Fleetfootwork or Hail of blades, both of which give him either more survivability and ms or more damage and ms than conqueror. Vayne, twitch, and ashe are all ridiculously weak early or have the same issue as trist where they get bullied out, making fleet for surviving the lane far better. Kaisa and varus both do mix damage conqueror helps with none of that for them. Furthermore, Varus's motus operandi right now is to not AA and instead poke with Q. Meaning he never gets the chance to stack it.
Jinx likes the MS and sustain from fleet even with it nerfed.
Also Lethal Tempo is garbage and very few ADCs actively take it over fleet or even press the attack which works equally well on both melees and ranged and provides extra utility in teamfights. Instead of requiring 3 seconds combat to proc, it only requires 3 AAs that have damage already added to them and the affect afterwards lasts 6 seconds. Which is exactly how much time a teamfight would need to last just to get two procs of Conqueror off as an ADC.
Regardless of how you feel, fleet or PTA are still better for ranged champions than Conqueror and your misconception that riot introduced it for ADCs is completely inaccurate. It is intended for melee bruisers and juggernauts and was specifically release with the statement from riot saying this.
