I'd tell you its because his builds are super flexible and he benefits from pretty much all ADC items. He can go full dash and build Essence or go early power with a more smoother build with Youmuus. He is a self dependent adc who doesn't need a support with him to succeed. He has good waveclear, aoe, single target dps, all depending on his build. He is a very versatile adc who fits into pretty much any comp and can even have the comp built for him.
It is not simply because he is really strong right now, but his ability to be what the team needs. He has always been high pick rate since Riot changed him to what he is today. As long as he remains this way he will always stay relevant when it comes to picking an adc.