The problem facing them is that Japan is an island both literally and figuratively. After World War II, the Japanese government created the class as a way to make car ownership more accessible. The tiny engines generally meant better fuel economy to deal with the nation's expensive gas, and the tax benefits also helped. It's made the segment hugely popular even today, with kei cars making up roughly 40 percent of the nation's new cars sales last year, according to The New York Times. The downside is that these models are almost never exported because they aren't as attractive to buyers elsewhere (if indeed they even meet overseas regulations). So if an automaker ends up with a popular kei model, it can't really market it elsewhere.
The government now sees that as a threat to the domestic auto industry. It believes that every yen invested into kei development is wasted, and the production takes up needed capacity at auto factories. The state would much rather automakers create exportable models. To do this, it's trying to make the little cars less attractive to buy, and thus, less attractive to build. The authorities recently increased taxes on kei cars by 50 percent to narrow the difference between standard cars, according to the NYT.
If kei cars do lose popularity, it could open the market up to greater competition from foreign automakers. Several companies complained about the little cars stranglehold on the Japanese market last year, but since then, imported car sales there have shown some growth thanks to the improving economy.