Because Heimerdinger is objectively oppressive against (almost) every melee champion, and is one of the least interactive champions in the game.
He's basically Malzahar, but if Malzahar didn't have to even press E again after the wave died. A boring, dead lane, that just feels like shit, and forces you to farm under turret, while he gets a lot of passive DoT. (Yes, even though his Q is technically an active, it's a drop and forget. There's no skill of any kind to it, no special quick activation, it's just press Q and forget about it until you have to move it again).
Zoe, however, was nerfed in a lot of different ways (Small nerfs to all 4 abilities). On top of that, she's just less oppressive for everyone. Melee? You can bait her E, and then engage. You can actually do something against her as almost everyone. (Not super immobile champions like Ashe or Mordekaiser, sure, but everyone has a counterpick).
Besides, only the flat damage on Heimer's beam was reduced. The 70% AP scaling is the same, the Liandry's 2-4% max hp burn is the same, the Rylai's slow that stops you from ever touching Heimer is the same. The only thing that was cut was the overly-high flat damage.