Lifesteal is one of the four-piece sets. That's by design, to force you to choose from the different 4-piece options, as they all provide unique benefits that the 2-piece sets don't. The 2-piece sets typically just provide an increase to one or more stats. That's not to say the 2-piece sets are bad - just that being able to get two of the 4-piece bonuses would make them too powerful. Picture someone running Lifesteal and Regeneration for example.

Krama nailed it right on the head.