Athene's Unholy Grail costs 400 more gold and only provides you with extra 25 Magic Resistance and sometimes more mana regeneration (depends on how much mana you are missing and/or if you score a kill/assist), while Morellonomicon gives you 20 extra AP and is easier to build, due to Chalice of Harmony costing 1000 gold.
Most people simply consider getting less AP and a little more mana regen/MR, simply not worth the extra gold and the loss of 20AP, especially in the early stages of the game.
Most people simply consider getting less AP and a little more mana regen/MR, simply not worth the extra gold and the loss of 20AP, especially in the early stages of the game.