Nobody gets Maw because it's not near as gold efficient as Hexdrinker. It's literally only ever gotten because it's more slot efficient than Maw is.
Zed and many other AD assassins get BC because AD assassins have few useful armor items (GA and Tabis being the only options, and GA isn't really useful if your team isn't able to save your ass anyways, and gives a shit amount of armor ), so instead they buy the next best thing: Health. And that health combines rather nicely with
. And it gives CDR and AD on top of that health.
You also have to keep in mind that BC is still a defensive item for most AD assassins due to their typical build consisting of AD, CDR, Lethality, AD, and more AD. Sure, it's an aggressive item for top, but that's top would be reducing their total defenses for BC's damage, while assassins are reducing their total damage for BC's health.