No one can say for sure, but off the top of my head I could think of some possibilities.
One would be that it may take longer to do a Vero resale. Therefore, they would stay in the listings longer eventually building up more than the WDW resorts that experience quicker turnover.
Another factor may be the increase in dues at Vero. Some folks just may not have the cash to cover the increase. People who bought "cheaper" at Vero never intending to stay there, may not have been prepared for the substantial dues increase.
And, of course, there's the general time factor. Vero has been open for quite awhile and is probably experiencing the lifestyle changes sales such as divorce, death, change in status, etc.
Just my thoughts.
I just looked at the Timeshare Store listings. I see 8 Vero resales and 7 Hilton Head Sales and a mix that even includes a VWL sale. I don't think there's any rush to unload Vero -- just looks like normal turnover.