First, welcome to FT.
Second, as you are probably already surmising, MVCI award availability is not the same as hotel room availability. Once a property is sold out, Marriott doesn't own the "weeks" there, the owners do. If they use their week, Marriott can't offer it for paid or award stays. If they don't use their week, Marriott can't offer it for paid or award stays either (its the owner's week and they can use or not use it as they want.)
An owner could offer up a week for rent (or ask Marriott to do that for them). In that case, Marriott can offer it as a paid stay, but not an award stay as the Marriott has been contracted by the owner to rent the week. The owner could trade their week into Marriott for points. If they do that, then Marriott gets the week and can do what they want with it (try to rent it, offer it for awards, etc.) At some MVCI properties, it just doesn't make much sense to trade your week in for points. At others, it might be more appealing.
So some properties will almost never have any award availability (the property is sold out and the owners will never trade their weeks in for points). At other properties, during off-seasons you might see some popping up. At others, like Orlando, there is so much timeshare availability plus the initial costs and annual maintenance fees might provide more year round award availability. It just depends on the property, its location, costs, and the season.