There are a number of factors for that
Kamiko seems to have done very well for Flyhigh Works, as they’ve just announced that it’s sold 200,000 copies on the Switch eShop. That’s just one platform, and the studio announced that it’s currently the third-best selling indie game on the entire eShop.
Kamiko‘s success is for multiple reasons, a lot of which is timing. Flyhigh Works dropped it on the eShop far before it reached saturation levels at nearly the start of the Switch’s lifecycle. It was also $5 (it still is), which made it an instant impulse buy, and it was a pretty good game.
To celebrate, Flyhigh Works is bringing their Fairune Collection to the Switch, which includes Fairune and Fairune 2 for the 3DS (both combined are roughly $10 on the 3Ds eShop).
Flyhigh Works Celebrates Kamiko Sales Success With Fairune Collection Announcement [Nintendo Life]
