Warlord's Bloodlust makes other Lifesteal sources unfavorable.
As title says, the mastery Warlord's Bloodlust is simply superior in terms of situation when opposed to other life steal stats.
Since the only two life steal items favorable by Marksmen are Blade of the Ruined King and Blood Thirster, neither giving critical strike, they are just very poor choices when given to many critical strike options that can also give lifesteal from Warlord's.
Tl;dr Critical Strike also gives lifesteal with Warlord's, so items like Botrk and Bt seem like bad choices.