Because most of the successful or even semi-successful games are mapped to Warcraft, and in the meantime, we do not have the Clash Royale's map.
First of all, Pyrogasm is right. Who would want to remake Clash Royale into Warcraft 3, and even if you do, why would it be a dead game? Why not something like, Starcraft? Sure, gonna take lots and lots of time, but at the end, worth it. Instead, you are using to revive a dead game, that you can install for free on the app store. Second of all, most of successful games that are mapped to Warcraft 3, are only done because they are still relevant and most of them aren't free or take up lots of space into someone's computer, while Clash Royale is the opposite of this. It's basically an irrelevant piece of shit game that's free to install from the app store, unto your phone and mapping it to Warcraft 3 is a complete waste of time.
Furthermore, I'll say that this project is bullshit and you can do better. Map something into Warcraft 3 like Warcraft 2, it's done in the hive so much times, but you might make a beautiful and fun campaign, not Clash Royale.