I'm inclined to think it's because of the Treehouse that Nintendo was ignored. Nintendo is now competing directly with media outlets for coverage, and during the most important event of the year, no less. It would make sense that the strategy going in is to either snuff out Nintendo or simply not compete with them because Treehouse will have the better coverage. Nintendo is walking a fine line here, it's pretty nuts. I'm just hoping we've come to a time where social media and word of mouth can spread news faster and better than games media.
When you have a list of best Wii U games at E3 from IGN that looks like this:
Super Smash Bros.
Mario Maker
Yoshi's Woolly World
Captain Toad Treasure Tracker
Xenoblade Chronicles X
It's perpetuating an idea that Nintendo did not show unique titles, mature titles, or anything out of the ordinary like Splatoon, Bayonetta, Devil's Third, Hyrule Warriors, etc.
Yeah it's nuts but like you said, I think it may work out for them. It's going to be tough tho. A lot of gaffers probably don't think of themselves as super savvy or out of the ordinary but you'd be surprised at the numpties out there that are easily fooled. It's how rubish gets spread around. A lot of people will take everything these outlets report as gospel and that includes the lack of news which can be pretty damaging in terms of mind-share in and of itself.
Overall however I feel Nintendo realised relying on such a fickle outlet to create and distribute your message is unreliable. It's ballsy and risky and could burn bridges but I still feel Nintendo doesn't mind being inclusive. Letting others stream the Digital Event and having games like Devil's Third revealed on IGN and even round tables helps keep some form of willingness to work with the likes of say IGN. It's almost like Nintendo is teaching everyone else how it's done, spoon feeding them like babies. With great power comes great responsibility and all that.