I suspect she doesn't have the inherent "screw you" mechanics to compete with Yi and Trynd for melee carry (less damage and ignoring slows than Yi, less invulnerable than Trynd) and her ult isn't strong enough to make her an ultbot in the same realm as Katarina, so I think she's relegated to being similar to Riven, a melee damage dealer who has to build fairly tanky despite that. At least she has the inherent damage to get away with building some bulk.