Meta Tanks tend to have very powerful Mid Game Power Spikes, and by that time most of the ADCs don't have enough damage to DPS them down.
So, some ADCs opt for an Early Game build path (hint: Lethality) that allows them to be active much faster than the casual Crit ADC build. Even with this build, they don't really destroy the Tanks, but they destroy any squishy champion in their sight.
So Riot wants to help the classical Crit ADC build by making them active against the role they are supposed to counter much faster. ADCs tend to build Last Whisper after 3-4 big items in general, now they can just buy Last Whisper a bit earlier, buy whatever they want, and upgrade it later.
They also lowered its AD, so it will be good to buy only against champions that build Armor.