Many Dem primary voters made up their minds over a month ago

[TheDormouse]

if exit polls are to believed, and if New Yorkers are similar to voters in other states in this regard.
http://www.nytimes.com/interactive/2016/04/19/us/elections/new-york-primary-democratic-exit-polls.html

So any campaigning going forward is basically to motivate people who have already decided on a particular candidate to actually get out and cast a vote. (Or to try to convince the opposing candidates' supporters that it's not worth bothering to cast a vote.)

There are very few left to be persuaded to make a choice they haven't already made.