It might be because that'll be over spring break. I've been to WDW a lot over Winter Break and Spring Break, and they usually have 2 parades. They're probably expecting big crowds for that time.
Great, it is always really crowded on Main Street as we usually leave around noon, add to that a bazillion people trying to sit on the curb to get a good spot for thre parade. Another reason to try to avoid MK when we are there in April!