[RedDevil]

The problem with kei cars is the size.

Any car needs to move the driver, in reasonable comfort.
Most cars need to move a family, even kei cars.
Due to their small outer size there is little excess space to work with.
They need to be very cleverly designed and built to be able to meet the requirements in reasonable comfort and safety (yes, the Japanese do think you really need at least a basic level of safety).

It would be much easier and cheaper to build the same car slightly bigger.
Just letting it bulge out at certain points makes for better aero and crash safety without even adding weight.

The only reason why the Japanese build kei cars is their severe lack of parking space in the cities, which they try to solve by making minute parking spots only fit for kei cars and tax reductions for kei cars.

So, kei cars are relatively expensive despite their size.
Without the parking woes even the Japanese would not buy them.