According to an article from a Japanese showbiz news site named Nikkan Taishu, in which the site did an interview with someone familiar with the situation, Sega wanted to publish Lost Judgement on PC/Steam to further extend the series' audience but the protagonist's actor Takuya Kimura's talent agency refrained them from doing so due to their very strict portraiture right policy (very limited pictures of their talents even on their website). Sega thinks this doesn't look reasonable in terms of business and decides to end the series with the second entry.
Most of the above info is in the 2nd page of the article.
Edit: Some of you ask why the talent agency prevent Sega from publishing the PC version, as I say in the post, the talent company "Johnny's" have a very strict portraiture right policy, they don't want their talents' portraits on "a PC platform that is not a game console platform and connects directly to Internet."
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