Nintendo of America is apparently to blame
SteamWorld Dig is hitting the Wii U on August 28th, and if you happen to live in the EU, AUS, or NZ regions, you'll get a 25% off discount for owning the 3DS version. But wait -- why isn't this cross-buy discount available in North America too?
According to the developer, "NOA (Nintendo of America) does not have this mechanism in place yet, so a cross-buy like this isn't available to our American customers. We've tried hard, and it's taken some time to get to this place...half a victory at least!"
This is the second developer to confirm the situation, as The Game Bakers' Squid's Odyssey also had a similar promotion for the EU and not for NA players. If you want a Cross-Buy solution in North America, be vocal about it to NOA.