Discourse doesn’t allow downvotes by design / philosophy. You can like but not don’t like.
For this reason, reactions are treated as likes with the exception of “neutral” reactions admins can choose but I suppose most communities don’t or rarely use them.
Boosts allow negative comments and negative emojis which seems a bit antithetical to discourse’s philosophy.
If boosts are tied to reactions, it might make people less inclined to “negative boost” (especially if the text field doesn’t allow emoji…? 
).