Mostly because a lot of items can serve the same purpose while offering better future use, such as Tranquils+Soul Ring offering you decently good early game, regen and mana, but the boots can be disassembled and the Soul Ring is cheap. Many of these alternatives, including Tranquils+Soul Ring, are cheaper than Vanguard as well.
And heck, a PMS+naked Vitality Booster+Ring of Health offer about the same, while also allowing you to still build directly into Battle Fury and Heart.