Only up-votes on posts and comments are federated. Is there a technical reason why (limitation of ActivityPub?) or is it a social reason?

My reading of the Activity Streams spec is that while ‘Like’ (used for upvotes?) is a standard Activity that all clients understand, there is no ‘Unlike’. But we can define new types of Activity:

The Activity Vocabulary defines a small number of Activity types that are common to many social Web applications. This specification stops short of defining semantically specific properties for most of these activities. External vocabularies can be used to express additional detail not covered by the Activity Vocabulary.

… implementations are free to introduce new types of Activites beyond those defined by the Activity Vocabulary