I'm not sure if poor sales are the reason since Mangagamer only releases hardcopies of games that sell well and 2 of the 3 Lilith titles have had hardcopy releases, although this might not necessarily mean good sales from Lilith's perspective. I wouldn't be too surprised if piracy was the reason why they stopped licensing games since many Japanese companies label Westerners as pirates because piracy is more common than in Japan. However, a big reason why that's probably the case for hentai is accessibility since very few titles get licensed, it's difficult to import adult goods from Japan, and some Japanese companies will release their English content on Japanese websites like DLsite which might not be as accessible or well known to English speakers.
I also feel like Mangagamer seems to be shifting away from publishing shorter Nukige to publishing longer games with little sexual content and more input from the player.
It's a shame that more content from Lilith isn't getting published in the West since I'd definitely pick up a copy of any of the Taimanin Asagi games if they got an English release. Not to mention that when I first saw Lilith releasing games through Mangagamer, I was hoping that it was a prelude to some of the anime adaptations of their games getting official releases as well. Even though it's roughly decade old now, I'd kill for an English release of Taimanin Asagi's first anime adaptation.