I think WotC has been pushing storytelling over the "game" for a while now, and I think that harms the popularity of this style of play.
If a group wants the best "narrative" they are going to approach the game in different way. We don't have novels authored by committee or randomly generated from tables for a reason. So as WotC pushes this type of game where the story telling is front and center, and you get new players exposed to Critical Role's tight narratives, the player base moves towards those types of stories. We see this is the growing sentiment, on social media, that DMs are entertainers and not players.
Another poster articulated the idea of open world games, where you have a overarching narrative that is center to the campaign but the players are free to move around the world and interact as they wish. I think this is where a lot of newer players view sandbox games. They think of the idea differently, as they started playing in a different time. This, also, lines up more with the actual plays you see on youtube which influence millions of players.
I also think Ezekiel said something very astute here;
It's just part and parcel of the specific design philosophy 5e has gone for
5e's core design lends itself to a different style. Sandbox, in a traditional sense, is possible, but not greatly incentivized in many of the design decisions of the system. I think this is why there is a divide in the perception of 5e. Certain playstyles expose flaws 5e's design, while others don't. The focused narrative games we see in actual plays simply don't have many of the issues with 5e's design as other styles might.
I think WotC decided storytelling was the part of the hobby with the broadest appeal, and leaned into this idea of narrative driven campaigns with 5e. And I think, with the help of Critical Role, the community, mostly, followed.
So I think it's a, somewhat, purposeful move by WotC. And I think they feel rewarded for that move, rightly or wrongly, due to how 5e has panned out sales-wise.