I love the series, but there must be SOME reason the games don't sell even after multiple decades and lots of hype.
It's actually quite a simple answer. The series has always been a very small, quirky, indie Japanese title. It's aimed for a specific audience, and not mainstream appeal. That's always been the charm behind it.
It wasn't until Sayonara that the series finally made it westward, and even then it only got digital releases. Fresh is the first title to actually see a physical release, but make no mistake the series is unbelievably niche. A hard-as-nails physics platformer with a very acid-induced level design vibe to it that was Japan only is far from having any kind of hype, unfortunately. The most attention it got in recent years was because of GameCenter CX, and even that isn't a huge degree compared to most AA releases. That was also quite a few years ago now.
The size of this forum says everything you need to know about the series. It has always been focused on being a very niche product, and that's been a lot of its charm. Fresh aimed to branch out from that, for better and for worse. I've said this before on a different post, but this is probably the largest the series has ever been globally. With Blade Strangers, Fresh, and BaZooKa coming out soon, this is the most we've ever seen Umihara Kawase out there. Even then, if you were to ask the average gamer who Umihara Kawase was they'd have no clue, much less any idea of the history.
It's not that the product doesn't work, it's been very small, limited releases sporadically over the years that have largely been in Japan only. Even in Japan, the product has never had a large success. Some games just don't become mainstream successes. Some just are simply what they are. Umihara Kawase, as she is, will never be one of those games. It's just not going to happen, not in her current form. and that's entirely alright, because her adventures are cherished by those that find enjoyment and charm in her games.