This could be a planned exercise by the democrats get more free publicity and get better polling data to select the ideal candidate with the best message for the run in 2020. 20 candidates get 5 times the publicity, (all of it left wing), than just 4 candidates will. Keeping them all in will allow them to slowly shift the fringe elements into the popular message, while keeping the total voter masses energized and continue to get large amounts of polling data.
If they are not taking big swipes at each other, they are not yet ready make a real push,,, watch to see when they go for each others throat!