It would be neat, but it would also require a lot of prep work on their part, all while likely delaying the games' release. I'm not sure they think it's worth it.
I don't see why that would be the case. They would just be taking the E3 Demos and putting them all on one app. It wouldn't delay anything.
E3 demos are notoriously unstable: crashes, bugs, and other major technical defects are common, even to the point of occasionally bricking consoles. Simply put, the things are too far from a finished product to be reliable, even though the publisher pulls small teams away specifically to craft these demos.
That's the reason why they limited them to Best Buys, with trained presenters, last year. It would do a lot of harm to release those same demos and have the public experience those defects without the filter of a Nintendo employee to both iron out problems and explaint the situation. Doing what you propose would require them to create demos that are good enough to be released, unsupervised, to the public, and that involves a lot more work.