And our understanding of the semantics at the time when we made the APIs. And backwards compatibility constraints. And whatever "feels" natural to the person who implemented the API.
Where it makes a lot of sense Elasticsearch maps the HTTP verbs to useful things. But when it doesn't make a ton of sense we just go with whatever verb feels good rather than trying to be super strict about REST. Also, we don't do linked data, instead relying on you to build links from context. I'm told that is particularly non-REST. But it is what we do.