At the time, the Charizard in the set wasn't running out the gate as a $600 card. With Hidden Fates you where able to get the shinies in Japanese form at cheaper, arguably the demand for it was smaller because the japanese version was a cheaper route.
This is brand new, and doesn't exist (yet) outside of US Champion's Fate.
I can't speak for the meta since I quit playing Pokemon some time ago (But I still like to collect) so I don't know what cards where playable from Hidden Fates VS Champion's Path, so my assumption is purely because the Shiny Charizard exists.
I fully believe that if that Charizard wasnt in the set, there wouldn't be as much demand for it (save for the Elite Trainer Box). Yes it would have still sold out, but not as quickly. I feel this can be backed up because pre-orders for Champion's Path products not yet released are sold out as well.
I'm trying to disregard my toxic hatred for Charizard, but I can't believe people are desperate for Gardevoir or Drednaw. It's all about that money card.
(this is also noting that I was completely oblivious to Charizard VMAX being in this set too)