A group of marketing materials for Earthbound was recently picked up on eBay by a gamer known as "Jesse" and, like a good gamer, he shared his good fortune with the internet. Once of the more interesting pieces includes a retailer book with information about the upcoming game to encourage stores to carry it.
As you can see above, one of the encouragements used was the promise of a $2 million marketing push aimed at a youth demographic. That's a lot of scratch for them to have spent on a game, particularly back in the carefree days of the mid-nineties. That's also a lot of money to get burned on if the product doesn't move. Earthbound didn't.
Of course, things might have changed where the actual campaign was concerned. It's entirely possible that Nintendo never spent near this amount when it came right down to it. But, if true, that would certainly explain all the reticence the company has shown towards localizing and releasing other titles in the series for a North American audience.