Keyboard with kana is inconvenient because it is just latin 26-keys keyboard and Japanese has about 40 kana characters. If they have added an additional row then it would allow typing twice faster than on a latin keyboard (because one kana character is equivalent to 2-3 latin characters).

But keyboard manufacturers do not want to make better keyboards for non-latin countries, they just overlay foreign characters on standard 26-key keyboard because it is cheaper.