I can't convince anyone in New Jersey of this...but the fact is many portions of the country aren't tied to a religious holiday at all. Some have trimester formats. The warmer portions of the country (Florida) end their calendars around 5/1. And remember that colleges also are all over the place - not a huge driver of travel to wdw but I bet quite a few people still do use that window to travel.
It's really covers a lot more than "Easter". But like I said - I try to convince my mother in law of this and she doesn't believe me...but she also is clueless that they are no more than 15% Catholics everywhere west of Philadelphia...even though it's the truth.
Anyway...there's a reason why 2/15-5/1 are never classified as the "slower" seasons on the Hotel rack charts.
As far as the NYC schools and why it's tied to Passover...here's a hint: ITS THE TEACHERS