I wonder who in Nintendo made the call to do this. You figure a big corporate suit would feel that an old NES game doesn't matter. This won't make a noticeable improvement in Wii U sales and won't sell millions of copies so who cares, right? But it earns goodwill with fans and just makes the Wii U that much better of a product. Whoever decided to do this knows that and also knows enough about Earthbound to know that for some of us this is a huge deal.
Probably Iwata since he's good friends with the creator of the series and was even personally involved in the develop of these games as well. Hell, I wouldn't be surprised if we got this game because Reggie and Iwata were having lunch last week and Reggie said, "Man, why do these losers keep asking for Mother games, who the hell would want to play that ****". Iwata then becomes enraged and goes "Reggie you bastard, you told me nobody in America cares about Mother for the last decade."
He then punches Reggie in the face, flies to NOA headquarters and takes the copy of Earthbound Zero that was locked away in the underground vault and uploads it to the eShop right away to personally right this wrong. That is why Iwata isn't at E3 because he flew back to Japan to personally begin the translations of Mother 3 and so it'll be ready for release later this year.