IT sounds like a big reason for the crowds are the extended school breaks in Florida school districts. The fact of the matter is - while breaks in Northern states have some moderate effect on crowds, they are rarely going to make WDW seem overly crowded. What makes big crowds in WDW is the calendar for Florida and also the surrounding states. (Most Georgia and South Carolina but also Mississippi and Alabama to a lesser degree.) For instance, we live in the NY and our kids don't start school until after labor day. Going to Disney the week before labor day is very low crowds, because even though many school districts in the North haven't started yet, the Southern schools all have. The same goes for Presidents day week - while the weekend itself is fairly busy, the week of is really not bad, because even though many Northern schools are off, southern schools are not.
The fact is, Disney attendance is still dominated by people that live within a 8 hour drive. These are people that can still take a fairly affordable trip to Disney, and therefore there's a broader range of income families that can afford these trips. (A family of 4 from Chicago has to add $1500 or more to the trip cost just to get there, not to mention rental car / parking / etc.)
