I think competition has a lot to do with it. If you want to buy a WDW timeshare, DVC has the absolute prime locations. Everybody else has to find other ways to compete.
When it comes to VB, DVC has to compete with other resorts, timeshares and rent-by-owner condos all up and down both coasts. And a lot of the prime-est of the prime locations were long gone by the time Disney started to look for a spot to build. There's lots of better beaches in Florida, and more appealing towns, too.
They had the same problem at Hilton Head, and that's why your 10 minutes from the beach. Disney obviously would have preferred to be on the beach, but Marriott and all of the condo developments got there first.
So it's Disney that's at a location disadvantage offsite, and that has to find other ways to compete. They haven't been all that successful, IMHO. I like the resorts myself, and they're a good use of my points. But if I wasn't already a DVC owner and wanted a beach vacation, I wouldn't go to VB. And if I wanted to stay on HHI, I would probably rent one of the Palmetto Dunes condos on the beach.