I think I may have missed the memo, but why was "This week on GOG" cancelled?

Because Google is Eeeevil!

edit: "The last episode, by the way. With all the unfortunate changes (a.k.a. "improvements") being introduced to YouTube all the time lately, the number of views we can get doesn't really justify the amount of time and resources we dedicate to the show."

You did indeed:
https://secure.gog.com/forum/general/this_week_on_gog_episode_45_the_fat_lady_sings/page1

Thanks. I get their reasons why (Youtube continuously gets worse and worse with the "improvements") and I suppose they never really got all that many views to justify producing the episodes. Still, I really enjoyed them and they will be missed!