I can't comment specifically on your choice of games, but Japanese computer games (in general) were primarily a result of the hardware limitations of the PC88/98 and their competitors. As NEC dominated the Japanese computer industry, it was only natural for game developers to create games compatible with their systems. This was much like how US computer game developers eventually started optimizing around Windows (hence the dominance of computer games on the Windows OS compared to say, Mac OS).

The thing was that NEC, though they were able to display contemporary advanced graphics, had very poor video memory. Part of this was due to the fact that unlike systems that utilized the Western alphabet, the Japanese computers required the display of Japanese characters. Japanese characters are much more difficult to read in low resolutions than Western text. This essentially forced the Japanese machines to run in high-resolution constantly, so although the graphics were good, animation was far more difficult as you could only use 8 colors. This not only contributed to the signature Japanese early PC game visual styles, but also meant that games dependent on animation (such as first-person shooters like Doom and Castle Wolfenstein) were much more difficult to make for the PC88/98. As a result, the most common games made were either static turn-based strategy games (Koei's Romance of the Three Kingdoms being one of the more well-known foreign ones) or adventure games/visual novels, as neither of these required significant active animations and were thus much easier to run on an NEC. Over time, these styles of games became the norm and subsequent games capitalized on these models, including, inevitably, more adult content.