I'd guess that it's like a lot of other "niche" goods (like keycaps for example). It may be successful and well received but that probably doesn't mean it was worth it for them to buy all the in house tooling to produce the set.

Rather it often makes sense to pay to tool a factory for a limited set of runs, produce the amount you know you can sell plus a bit of extra inventory, and then eventually when it sells out you can consider tooling up again.