I started following the manga when it first started releasing. Based on the artwork, environmental storytelling, and other small details, it's clear the author really cared about the world she was building. I can only speculate that she created a lot of backstory and lore that just never made it into the main series because it didn't fit the pacing of the story.
I think it was just time to end it. The main series ran for 12 years. That's a long time for a series that, as much as I love the art and characters, doesn't actually have a very deep plot. The final arc was actually announced a year before it ended. So I think it was less about sales and more about the author wanting to close out the series on her own terms, while still releasing these extra chapters so she could put out the side material at her own pace. You can tell the artwork in these extra chapters isn't nearly as detailed as the main series, which fits that theory too.