From my discussions with Drop, I'd guess they are unpopular.

I've been trying to talk them into selling color sets piecemeal for a while now, like selling just a set of White on Red modifiers, to go with my BoW alphas.
Instead, they have chosen to release a bunch of horrible off colors, like the Iron Man and Captain America kits, neither of which has an option for the White on Red modifiers I have been wanting.

I think they would have been much better off creating generic kits in Blue on White, White on Blue, Red on White and White on Red.
That would have let users create their own "Captain America" sets, using their own chosen mix of colors, while letting Drop get away with paying royalties only on the Artisan keycaps.
That would also have allowed me buy the White on Red modifiers I have been wanting, as a standalone set.