Simple. Zelda Wii U will try and be the best game of all time, and Xenoblade X just never was quite at that level. In what way is that not more ambitious?
The obvious points about how Zelda will compete a level above X are the clearly superior graphics, undoubtedly superior audio, and more dynamic gameplay in an open world. Making a game like Zelda in a seamless world is much harder than making a game like X in one.
I'm sure X will be a great game (or perhaps I should say is a great game), and in terms of things like customisation and what have you it of course aims for more than Zelda - a series that isn't about that.
I guess my point is that X may be considered "more ambitious" in quantity of content, but if past Zelda titles are any indication Zelda will be more ambitious where it matters, in the quality.
