Why did they remove Boo?

[Croconaw]

And why did they not add Dry Bones back?

[Spongyoshi]

Boo are enemies in some minigames and Dry Bones is a boss already. 
I think they don't want any enemies playable if you ask me ^^'

[Colyton]

They're making mistakes and wasting potential!

[Vipsoccermaster]

Developer's choice. Blame them.

Boo is playable in Mario Shuffle, though, so not totally out of the game. So Boo isn't really removed from the game.

[Nintega Dario]

Boo are enemies in some minigames and Dry Bones is a boss already. 
I think they don't want any enemies playable if you ask me ^^'

It's really dumb in my opinion, they could've made the Red Boos NPCs...

[YoshiGamer16]

I feel that in Star Rush, it's because they only wanted nice characters for the Ally mechanic's introduction. I am just glad they are making their triumphant return for Super Mario Party!

[Spongyoshi]

Same!! I love that new roster! (Even if I'd love Toad & Toadette but oh well...)