I've been watching a couple streams, it basically comes down to this : Shiv is, in no way, better than PD, but :
-Shiv's cost is much lower. Adc's have a lot of items to buy to reach their true power. Buying Pd puts your build behind, as the stats you get are not valuable enough early/mid game for the 500+ gold it takes.
-IE rush is the meta, but it has a horrible build path. A BF sword is great and all, but it costs a lot of gold. If you're behind or forced to back early, odds are you won't be able to buy it. So you buy a Pickaxe. But then the enemy adc, being ahead, comes back with a BF, and keeps zoning you out of minions, so you back early again, far from 1500 gold. At this point you basically give up your lane phase and just buy an Avarice Blade and farm for late-game (or hang on your gold and hope you don't die or lose out on too many cs, or call for a gank should the opportunity arise)
-Or there's the times where you're actually not getting stomped, and you manage to buy a BF on your fist back. At this point, 2 things may happen: Either you keep stomping lane, and on your next back, you come back with an IE and just kill everyone, either the match-up turns out to be pretty close and you can't finish your IE. Of course you can buy a Pickaxe, but Avarice blade has small advantage in the long run at this point.
See, Shiv is not just 500 gold less. If Avarice Blade made you 200 gold, it's now 700g less. If you made 400, it's now 900g less. The world where you have the perfect amount of money every back so you can go from BF>IE>Zeal>PD is so rare, that most of the time people just go shiv and if they ever get a full build swap it out for PDé