The reason I suspect they don't announce game content at events outside of official Nintendo sources is they want to control the information and who sees it--they can't guarantee the wider Nintendo/Smash fans would be at Minecon or even necessarily watching EVO, but they can guarantee that with Nintendo Directs.
And I know I feel like a broken record, but Verge doesn't leak stuff, he gives us rumors--we can only guarantee they are leaks after the fact, no matter how credible he is (which I think he is). The only thing close to a real leak we have is the Ken picture, and that's assuming it's true (while very likely, it's not guaranteed). The distinctions matter, as rumors could possibly be false/misleading, while leaks are directly from the source.
EDIT:
Only exception seems to be Ubisoft's E3 events for their crossovers.
Which makes sense, given that it's Ubisoft doing most of the legwork in both cases.
Agreed. We would probably never see a Nintendo game at another conference (as some had assumed with Smash content being at Microsoft's conference), but Ubisoft games with Nintendo content make sense outside of Nintendo's announcements. It's also why we would probably not see a Rayman Smash announcement at a Ubisoft conference--while it's their character, Smash is ultimately Nintendo's product. In fact, I recall seeing many Nintendo fans not knowing what Star Link was when watching Nintendo Directs, as they didn't watch Ubisoft's--that tends to be what they want to avoid happening.