Must read!! When is this coming out??
Carolyne Andrea Cremer had written a blog post back in February of 2016 regarding the future of this series, however due to the revamp of her site, there doesn't seem to be a blog there anymore (there is a link in one of the reviews below but it no longer exists). Hopefully this answer helps the questioner and whoever seeks the answer as to why this series seems to have ended with Snakeroot. The gist of it is: Soon after SnakeRoot and Captive (her adult Nightshade book) were released and while she was starting the next book for the Legacy series, she had fallen ill. She didn't go into detail about her illness, but it lasted almost a year and it had gotten to the point where she could barely stand by herself for very long. She got better, had started other projects and set a date for the next Nightshade legacy book in late '15, early '16. However, when ever she started work on it, she couldn't get into it. She'd been out of it for so long and her memories of the Nightshade world were sort of tainted by this dark time in her life when she was so sick. She didn't say she was going to quit the series forever as the characters are very dear to her, however, she had to put the series into hiatus for her own mental health, more or less.