The whole point of the kei form factor is different parking rules in dense, sometimes cramped Japanese cities. Due to the small footprint, they are allowed to park in more places.
These are urban vehicles, not intended for highways, except in a pinch.
The real reason is because everything is small in Japan due to a lack of absolute space. Remember, the place is a god damn island nation with mountains for its interior.
Roads are narrow, particularly in the countryside and especially if we're talking about roads crisscrossing between farms and rice paddies. Normal sized cars quite literally don't fit, much less normal sized trucks.
Consequently, out in the towns and cities you see far more normal sized cars because the roads are wider.
Who needs a cigar when one could read Wikipedia first?
> In most rural areas they are also exempted from the requirement to certify that adequate parking is available for the vehicle
Kei cars have a number of prescribed limitations (yes, because space is at a premium, but so is / was fuel, metal, etc), and even have special number plates.
Also, there are designated parking spots for them where other cars won't fit:
> Some places in the parking lots are smaller than others and usually painted with the character “軽自動車” or just “軽”. These spaces are reserved for the small “kei” cars
These are urban vehicles, not intended for highways, except in a pinch.