Something I've realised is important for vocal synth designs is how easy they are to fit into different scenarios. I can put Miku in just about any modern setting with minimal changes to her default design, and she fits right in.
But with a lot of characters it's quite hard to think of a background you can put them on without them looking out of place. With some, it's hard to imagine how they would even move or sit down, which really limits how much you can use their default design. With others, they have a specific theme that will only work in a particular scenario.
Since vocal synths are all about fitting your singer into your stories, a niche character design is going to limit the amount of fanwork that uses your main brand.