Because GC was axing the shit ideas like artifacts and randomly distributed legendaries that I'm certain were proposed in his time as lead and encouraging great QOL changes for the game. His team introduced systems that helped negate RNG (badges for one) instead of increasing RNG in every possible corner of the game. In other words, GC led a design team that made the game fun, accessible, and challenging at the high end all at the same time. Ion leads a design team that made mythic raiding a full time job, introduced boring as fuck grinds everywhere, and sent classes and specs back to ridiculous levels of simplicity and hid the good gameplay behind artifacts and having the right legiondary.
At least for my tastes, Ion and his team have not taken the game in an enjoyable direction for me and the frustrating part is that I feel like his team is making mistakes which were corrected by GC's design team years ago. For example, GC firmly believe that "increase damage by X%" type talents were boring, pointless, and really not a choice at high levels of play. Ion re-introduced an entire pointless, grindy, and very damaging (balance wise) tier of exactly that type of talent with paragon traits, which Legion would have been MUCH better without.
Just because Ion did a great job with Ulduar (and I mean that genuinely) doesn't mean he's suited to be game director.
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https://wow.gamepedia.com/Developer_Watercooler
I count 14 entries by GC, and he also used to actually go on the forums from time to time to discuss issues. He also did class specific Q&As which he personally participated in (at times). I pretty much guarantee those were FAR more effective than the PR stunt Q&As Ion does. So, GC basically invented the Q&A and Ion has now turned it from something useful to something useless.
Here's a quote about GC I found randomly from way back when he was actually the lead designer:
"For all the times I've disagreed with what GC has to say, I do respect him that much more for actually answering a lot of the communities posts. It's a shame some think that just because he doesn't answer their opinions or questions that he must be ignorant. I'm sure he reads but he's also a systems designer for the game. Posting on the forums isn't their first priority.
I can't wait for the Death Knight, Warlock, Warrior and Paladin Q&A sections. But I'll probably read all the classes regardless."