Ath'kethin
Elder Thing
I have to agree, though this trend actually started in 4e. Not all monsters need origin stories, and not all monsters need to be created by the same handful of epic-level bad guys (in Volo's it's primarily Orcus, the Queen of Air and Darkness, and Yeenoghu)
The whole "evil humanoids don't get to use the same gods as the rest of us" trope has been around for decades. What I dislike is integrating it so completely into the background and motivations of the monsters. Compare the section on gnolls in Volo's Guide to the section on, say, elves in the PHB. Elves have been associated with Cory whatshisface for decades, too, but there is no mention of the deity in the elf section in the PHB.
Now, the whole "driven only by their creator god's will" bit is prevalent throughout 5e; after all, the Orc and Gnoll sections of the MM have god-serving specific variants of the monsters. There was no reason to expect Volo's Guide to be any different. But I still don't like it.
EDIT: More to your point, I never played much 4e, so I guess I missed that aspect of it. I would have disliked that aspect there, too.