So, Prima, to expound on Rubick's short response:
Champions reliant upon their abilities can't function bot well. The reason being they lose experience from being in a duo lane. This in turn makes all of their abilities weaker by rather large numbers, makes their cooldowns longer, the list goes on. Unless the ability-based champion gets fed to oblivion and makes up for the levels they've lost, they will inevitably fall off.
ADC's tend to conversely be your champions that don't really need their abilities (or more accurately all of them) in order to do their job - at most, their job is front-ended into one specific ability and their other two abilities tend to be some kind of utility to help themselves so they're not completely helpless should they be caught alone. Since all they ever lose in this way are flat stats, they're the class that can easily make up for their weaknesses with items, making them the most appropriate to be paired in bot. And you have to have a pair of players some where in the game because there are only 4 lanes total and there are 5 players on each team.
So Ziggs falling out of bot was an inevitability. While it's not like he was the worst out there or even particularly awful, getting insanely fed is fallible - it's hit or miss. It can happen one game and then another you can be shut down to oblivion.
You then have to double this with the fact that right now, AP itemization is far, far, far more expensive than AD itemization (besides Morello), meaning a Ziggs bot would be shafted both out of his ability scaling and now he's not even scaling his items as hard as a traditional AD carry. Ziggs just wouldn't work in the current game climate unless he flat out snowballed beyond comprehension, and that would certainly be a feat of luck and just crazy happenstance.