Copying from Patch 3.10
Needlessly Large Rod and Deathfire Grasp have been removed from these map modes, with more mid-level AP items added in, including Liandry's Torment, Seeker's Armguard, and Moonflair Spellblade – a mid-tier defensive AP item. Blackfire Torch now has the same active ability of Deathfire Grasp.
This small-scale AP rework is intended to address AP survivability and viability in Twisted Treeline and Crystal Scar. Needlessly Large Rod (and, by extension, Deathfire Grasp) have been removed because it was unreasonably difficult to save up for on these maps. The addition of Moonflair Spellblade and Seeker's Armguard should help with early-mid game survivability.
My guess would be that Riot thought AP champions on the Rift are less in need of early game surviability. TT is much more about fighting and skirmishing as a team, meaning mages who are typically worse at that? need extra.