I suspect at least some resale buyers don't actually understand that Fixed Weeks are easy to opt out of, and so they are only a value-add. In the worst case, they are identical to a contract at the same resort without fixed week rights. But, if for some reason you think you have to use the week, then the market is limited.
I also have a sneaking suspicion that odd-numbered contracts also have a little bit of a headwind. Things that are not multiples of 25 (or, at the very least multiples of 5) are "odd ducks" on the market.
I bought a fixed week primarily as a hedge against availability problems. I expect to almost never use it as written, but figured it only gave me more options. I did not buy it for potential resale value. If I'd been doing that, I probably would have avoided RIV entirely.