I'm guessing that's because Barney & Robin has been done before, and ended pretty unsatisfying. Given how they broke up, how credible is it that they would get back together again? It'll also give the writers the opportunity to repeat a lot of old jokes, whereas with Nora there will be new stuff happening.
On the other hand, there are some strong signs that it'll be Robin in the end. When Lilly says the bride is asking for Ted, it's more plausible that it's Robin (seeing as they are close friends) than Nora (pretty much strangers). Even more to the point, in episode 3 (close your eyes now if you're not there yet) Victoria says at the end that the 'just friends' thing between the three wouldn't work and Future Ted confirms it. Seeing as Future Ted and his family are still in contact with 'Aunt Robin' and 'Uncle Barney', those two will have to end up together to make sense.
I think the whole "narrating from the future" thing is overdone anyway. It was cool in the beginning, but now it seems they give away too much and force lame jokes out of it..