When you book a property through the WyndhamRewards website, you can earn points and get WyndahmRewards elite benefits.
Well, look at this post in another thread here:
https://www.flyertalk.com/forum/31324527-post8.html
It says that Wyndham allows properties to keep their Wynhdam family brand name but opt out of the WyndhamRewards program!!!
So maybe these properties that aren't bookable on the WyndhamRewards website are one that have opted out of the WyndhamRewards program?
It would nice if WyndhamRewards would explain that online for each property, rather than just never showing availability, if that's the reason for never showing availability (while the hotel is easily bookable through other channels).