And for the popularity of these games, game companies make many mobile games, usually for non hardcore players. For example, Japanese game company Global-Gear has created many mobile casual games which use daily customs and institutions in Japan like college circle activities, young people's interaction in a shared house and romantic relationships between young college students.
These games are easy to be played because they don't require video game conventions and special knowledge about fiction like fantasy or SF. They could be called as "casual games" based on the definition of Jesper Juul. Juul explains 5 common casual game principles: fiction, usability, interruptibility, difficulty and juiciness (positive feedback).
I once analyzed the factors of the popularity of "Yo-kai Watch," the Japanese console game series created by Level-5 and explained the characteristics of its game mechanics by using Juul's framework.
There are few studies about Japanese mobile games and casual games. But we can research on them deeply by focusing on game, player and culture around the former two and using the tools of video game studies.