User blog comment:Just loves to play/Bloodthirster/@comment-620927-20130619061750

As far as sustain goes, building two Vampiric Scepters will give you more sustain than one Bloodthrister, making it a little inefficient as far as gold to sustain ratios are concerned. But Bloodthirster is great for its huge spike in AD and the snowballing potential, which is valued by Bruisers and AD Casters.

The problem being that Bloodthirster is kind of a one-dimensional item: That is to say, it covers sustain and gives you Tons of Damage™, but Black Cleaver and Atma's and even BoTRK provide greater spreads of stats that are gold efficient by their own right. It's more appealing to go for items that will give you more variety of stats at low, low prices.