Hi. I explained this in the release devblog. I wanted the last 3 Elementals to be a bit "rare" since the other free ones have been downloaded a few thousand times. Wind hashashin for example is downloaded 10,239 times as of now, so adding a price reduces that to just a few hundred, so they'll be seen less in other games, making them more "special".
But now that I think about it, I might change my mind in a few months/year once the set is complete. So I might make this free someday. The ones who bought will just have the advantage of using the assets first. :)