Probably easier to book the Javitz then (but that is a total guess).
Yeah, I think the NYCC organizers are the mercy of the Javitz Center. They probably couldn't get the April dates because the place is already booked.
It was like that with SDCC back in the early 90's when we first moved into the San Diego Convention Center. The dates were all over the place. Sometimes it was June, sometimes July, sometimes August. In 1996, the year the Republican National Convention was held in San Diego, SDCC was almost moved up to Orange County.
Now that SDCC is established and plays a such big part in the local economy, we're pretty much locked into the last weekend of July as the official date.
In time, the same thing will happen with NYCC. As the show continues to grow, the organizers will have a much bigger say-so on when it gets held.
(Next year will be interesting, with Toy Fair, NYCC and Wonder Con happening back-to-back-to-back. I wonder if DC will dis NYCC for Wonder Con and Marvel will dis Wonder Con for NYCC? Or will it be the other way around? Or will they attend both shows and have con fatigue set in? And does anyone really care about Wizard World L.A.? Or Wizard World Philadelphia? Am I asking too many questions? Bueller?)