Well Xenoblade 2 has sold close to 2 million copies while the previous games didn't even cross the million mark. Out of those two games, the first Xenoblade is the closest in terms of gameplay, structure and story. Which makes sense since the Wii game is literally Xenoblade 1, while X was meant to be it's own thing that would be different.
If Nintendo wants Xenoblade to be a longer running series, they want to give the newcomers to the series something they're more likely to enjoy to help build up the brand right now. This way if a port of X does eventually come out and it alienates some of the new audience, they would have experienced 1+2 first, so they're more likely to give another game a chance since the series is 2/3, instead of 50/50 like going from 2 to X might have caused.