Don't have the exact response, at the end that is Logic Apps team that decides.
But in my opinion, this can be because the service is on preview, let's say MSFT will launch a new cognitive service and also Logic Apps team will create this new connector, this connector will be in preview until this service go GA. Once it goes GA the connector should be GA also.
Or is simply a new connector or action is added, they will be in preview for a while, until the team decides to put it on GA.