....Why?

Not to mention that most of the indies that can only be found on Switch are exclusives to the Switch because of the devs themselves and not because of Nintendo.

Outside Cuphead is because MS, all other indie games which come to Switch is because Switch indie sales are very good, the decision of these games is from the developers, not because MS.  

Castle Crashers isn't a MS exclusive. It's also coming to PS4, and was on the PS3.

I dont think Nintendo needs to return the favor just because MS is doing this. MS is doing this to help themselves, Nintendo is just nice enough to allow it.

It is more of developer's choice, more systems, more sales. Is it on ps4 yet?

I dont see the point in locking small dev games to one console. It makes literally no sense. The more they sell the more popularity and profits they make. Xbox isnt just a console, its a eco system that will help increase sales.
Cuphead is a huge game for MS to let on Switch, be nice if Nintendo returned the favor.

Nintendo don't own those indie games, the makers of them can do whatever they want. Want those games on Xbox, you need to ask them, not Nintendo.

Why does it have to be a Indie 3rd party game? I mean MS gave Switch Minecraft and Cuphead and possibly more to come. Wouldn't hurt Nintendo to offer a low end IP or something back. 

Ill be happy if Nintendo add a MS character in Smash.

About Cuphead, MS want to do it, not Nintendo though, a Cuphead character could be in Smash.

About Minecraft, Sony should put a MS character in PS All Stars too, right?

Microsoft does it for the Xbox live accounts. Just like Epic wants more Epic store accounts.

haha, that's not how this works.