Anuke has their own reasons. On iOS, the fees to place it on the Appstore is fairly expensive and a long process, so to make up for it, it's paid on iOS (Free on Android though, since Google doesn't slap you when you want to put an app on to Google Playstore). The Steam version, while identical to the version here, has access to all Steam functions, such as Steam Play (removes the need to port forward), the Steam Workshop (for downloading maps and mods), and auto-updates (so you don't have to constantly check back here). It's also just a nice way to support the dev, and it doesn't even cost much.