In the good old days, if rooms were available you could book them.
Now computers are controlling reservations to maximize capacity and the spending habits associated with of the length of stay to maximize revenue.
For example, giving out two 3-day reservations could be more profitable than one 6 day reservation, so those are prioritized.
Disney has a ton of data on reservations and spending habits and no doubt they've got supercomputers that are looking at it from every angle to make another dollar.
With magic bands, they have even more data. Just think of what they could do. Big spenders mysteriously find lots of fastpass+ availability, while frugal types are left waiting in line as "punishment."
Perhaps a bit too cynical, but that's what they can do these days with computers and big data.