Back in January Bernie's campaign had suggested April 14 in New York.

However anyone who has ever seen a campaign knows that events can change many times over from Jan to now, two months is a long time in a campaign. Just because an event wasn't on the schedule back then doesn't mean Bernie is playing games, it just means at that time no event was scheduled. The Sanders campaign has a rally scheduled on that date. That is why Bernie turned down the April 14 at this time.

https://berniesanders.com/press-release/sanders-campaign-statement-on-debates/

PRESS RELEASE
Sanders Campaign Statement on Debates
JANUARY 30, 2016
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DES MOINES, Iowa – Bernie Sanders’ campaign manager, Jeff Weaver, issued the following statement Saturday on negotiations over additional debates:

“After refusing for months to participate in more debates, Secretary Clinton requested that the Democratic National Committee rules be changed and that a new debate take place in New Hampshire next week. We agreed pending an agreement on three future debates in March, April and May. Unfortunately, the Clinton campaign has not accepted debates we proposed on March 3 in Michigan and April 14 in New York. They apparently agreed to May 24 in California. The Clinton campaign, after not accepting Michigan, now says they want it. We are pleased to do it on March 3 before the Michigan primary provided the Clinton campaign will agree to Brooklyn, New York, on April 14. Why won’t they debate in Brooklyn? What’s the matter with Brooklyn?