The game's (very) last event was in January 19th to 31st:
http://syanago.com/news/#20170119_1
In one of their tweets, they said their finished with the game, without any reason whatsoever:
(Japanese)
My guess is that, the parent site of the mobile game, Autoc-One JP, weren't satisfied with the lack of popularity and had to pull the plug. It's sad because, they didn't had a chance to work with other game developers, like DMM (creators of Kantai Collection). That could've saved Syanago...
Plus, I understand what you mean. There are several Cargirls, which became iconic in GT. Like, the Mitsubishi FTO, Mazda RX-7 (only contains FB RX-7), and Nissan Silvia. (Sadly, no S14 Kouki nor Zenki.)
Full List of Syanago (Japanese):
http://syanago.com/wiki/?車なご一覧