There's another theory running around that Stanley cups are also growing in popularity due to a demographic focus of Mormons.

It didn’t take long before netizens began pointing to a connection between the popularity of the tumblers and Mormonism in the United States. For those of you who don’t know, Mormons are taught to not drink hot beverages, as they believe that “hot drinks are not for the body or belly,” thus avoiding tea or coffee and instead turning to alternative fizzy drinks for caffeine.

To keep them at an approved temperature, the Stanley Quencher’s ability to keep a drink cold for hours makes it a perfect option, thus making it extremely marketable to this particular demographic.

Source [blog]: https://screenshot-media.com/the-future/trends/mormons-stanley-cup-craze/

I believe the "hot beverages" claim is a bit misleading. I'm not a Mormon, but my understanding is they "hot beverages" only applies to coffee and tea. It was interpreted as a medicinal phrase (like how a "cold compress" might refer to a particular medicinal application of cold rags and not any cold rags?). The Mormon Church allows members to drink some cold caffeinated beverages since they are not "hot beverages". However, I think they weirdly still ban iced coffee despite it being cold...

Anyways, they represent a sizeable 2% of the US population. 6.5 million people who generally abide by these cold/hot beverages principles. So a running theory is they command a decent portion of the thermos market share.

I'm not an expert. I'm just sharing what I've heard.