If forum posts are any representative snapshot of member practices, some of this difficulty originated with economic struggles in 2008 and 2009. During the recession, members may have been banking their points in greater numbers due to job loss, financial insecurity and similar factors.
By 2010 and 2011 when confidence is recovering members are traveling again, using not only their current points but those banked points and perhaps even borrowing.
Without the impact of banking and borrowing, the resorts should be near 100% occupancy year-round if all members are using their points. The same is true if the numbers of banked and borrowed points are comparable.
But we are probably in the midst of a phase where there is an inordinately high number of prior years' banked points being used. The net result is resorts cannot accommodate the number of points members' wish to use. That makes reservations harder to come by--particularly for folks who are accustomed to a certain level of availability during their preferred travel time.