Fun story! I think the author is a woman, and her stories tend to end around this point just because the sheer pressure of romantic love is so intense it's hard not to move to the next step. It requires extreme dedication to "edging of the soul" to not just have a sex scene, and when a sex scene is had, the sexual tension mitigates.
This series would have been hard to transition to a kind of real fantasy narrative, which is what it would need to justify going longer than 40 chapters, much like the high-school love story.