To my understanding the use and popularity of kei cars in Japan is a thing for the same reason as massive cars in the US- favorable regulation.

My undersunderstanding, take with a grain of salt:

Japan created the kei class of cars during an economic recession to try and get motorcycle manufacturers to make super small, lightweight, affordable cars with spare parts in order to help the economy and make motorized transportation more accessible. They had a huge tax advantage for buyers.

Here in the US, emissions regulations that change based on the size and weight of the vehicle inadvertently resulted in SUVs and massive trucks being more favorable to manufacturers and less expensive to consumers.

Japan kinda seems like they've always had more appreciation for small cars, and america kinda seems like they've always had a bit of a preference for large heavy cars, and we could speculate why that is culturally (amount of space, road sizes, what the motorsports there have historically looked like with mountain driving vs drag strips and highways cruising, and all sorts of other reasons) but it seems almost misleading to leave out massive regulatory factors that differ in the two countries- intentional in Japan, and I think very unintentional in the US

For those who think kei cars are super cool (they are) hagerty did a really fun video on the exciting kei cars that enthusiasts enjoy, but it also kinda takes you through the conditions that led to their existence and then eventual popularity as regulations were tweaked. (But do know in advance that it's a car enthusiast's channel, and the man loves cars. I recognize not everyone here might enjoy his perspective on automobiles as much as I do)

Hagerty's mini-doc video on kei cars: https://youtu.be/LcB-nFt_bm0?si=tj-RquKZfwgUW0Uo

Edit: fixed a typo