Here is the Patch Notes info regarding the change:
"As Sheen's offensive AD upgrade, Trinity Force promises mid-game dominance through double the Spellblade damage of its component item. By contrast, Iceborn Gauntlet is the upgrade for champions that can take advantage of its utility. That separation’s been a bit fuzzy this season, with many toplaners picking Iceborn to straight-up outduel their opponent. While this was partially due to Iceborn being overtuned, we also realized Trinity Force was suffering from a lack of focus.
The best Trinity Force users reliably proc Spellblade on cooldown - Jax, Irelia, and friends wield equal blends of abilities and basic attacks to deal damage. That combat pattern is key: it means Trinity Force users don't rely on hyperscaling basic attacks to fight enemy champions. With that in mind, Zeal is a marksman-shaped peg jammed into a fighter-shaped hole. It fits... just not perfectly. Stinger is better suited for the job: attacking faster and casting spells faster squares with the goal of maximizing Spellblade uptime. With new focus, Trinity Force is better than ever at fueling the mid game pain train for champs that want to play the Spellblade game"