They revealed Cuphead during a random indie direct. I don't think Nintendo cares for "this is the perfect opportunity to do this" if they did they would have announced Erdrick when they spent 15 minutes talking about Dragon Quest the last direct.
There’s a few degrees of magnitude between Cuphead on Switch or Dragon Quest in Smash and Banjo Kazooie in Smash.
Smash is one of Nintendo’s biggest franchises of all time, and represents the greatest gaming crossover. Banjo’s a former Nintendo character now owned by Microsoft and a character that represents a major fan favorite in the Smash community.
Cuphead, for as great as the game really is, is also just an indie title backed by Microsoft. It’s popular and a big hit that is absolutely a big deal being on Nintendo, but it also basically got to headline that last indie Direct as is.
They saved Master Chief Collection on Steam for the Xbox conference as well.
Banjo in Smash would be such a fundamentally unique narrative in gaming that it’s one of the few things related to Smash I think would be intentionally timed for maximum effect. You have two of the biggest names in the industry not only actively working together, but actively putting one of their personal IPs in a competitor’s biggest IPs. That’s not the sort of thing that happens really ever. It’s a moment that will generate hype and good press if it happens. Same would go for like Master Chief in Smash. That’s an announcement with more weight than basically any other character I can think of. So I think Banjo would be the exception and the special instance where they pay more attention to his reveal.
Dragon Quest is less monumental given that Square Enix is already involved in Smash and the series has closer established titles to Nintendo at this point. Plus, the series doesn’t have the same popularity in the West, so it’s less of a big deal. Also, I know some people took victory laps when we didn’t get a DQ announcement when they talked about it in February. But given Joker’s current predictament, it would have made zero sense to have announced Erdrick at the time when he’d be months out still as well. And that would have gone for all other characters at that point.