It's about thresholds. Take a hybrid Blade of the Serpent UB build right now. It's very, very fragile early game, but you can get away with hit because spit does enough damage to drive away most players. Even up until level 5-8 a spit UB is still quite fragile, but the damage continues to scale up and balances it out.
The thing is the BotS literally gives you effectively infinite mana, and its STILL a risky item to pick against a pure HP stacking player. An effective buff to BotF (which let's be honest, that's all this really is in the end) simply makes BotS that much harder to use. Its relative strength declines.
The same idea applies to the other unique items as well, and don't forget, as the other people here have mentioned already, that priests and monk idols heal a % of total health and virtually every mid to late game fight involves sigils. Not only that, but unless they fixed it, priests/clerics/high priests/bishops all share different cooldowns, and good players will pick different idols to capitalize on that.
Let's take a fairly common scenario:
Sedna, Oak, and UB vs. whatever.
Sedna buys high priests, Oak buys Bishops, and priests have hit the field. UB has chosen spirit of the fallen and uses a sigil.
Spirt of the Fallen is, at that point, giving him 1500 HP. UB takes an opening volley from 1-2 DGs is then healed for 15% by priests, 19% by high priests, and 22% by bishops, for a total of 56%. That means Spirit of the Fallen, in the first few seconds of the fight, has already given UB 2340 health. In 10 more seconds she'll get 840 more from it when the second wave of heals hits her.
That's not a perfectly laid out scenario by any means, the damage and heals are likely to be staggered some, but my point here is that the way % heals and % health modification from sigils work puts a player who chooses something other than a max HP item at a disadvantage already. Their favor item must enable them overcome that disadvantage if it's to be worthwhile, when it comes down to it, and once these items hit the field that just won't happen the vast majority of the time.