I think Rockets fans overrate his trade value.
I think the NBA as a whole has gone away from the big plodding center that you can't run plays for and is mainly there for defensive/rebound purposes. I don't think many teams want to pay $15mm for that and finding an $8mm ending contract isn't especially difficult.
I think teams that have a rim protector, want a guy that is more like Ibaka, Chandler or Deandre Jordan that can move and is much more athletic than Asik. The league is just too athletic.
Plus, think about building your team to battle the teams you have to go through.
In the west, that is Tony Parker and Kawhi Leonard, Durant and Westbrook, Lilliard and TheMarcus, Howard and Harden, Paul and Blake, Curry and Klay and in the east it is Bron/Bosh/Wade, Wall and Beal, George/West/Hibbert, Joe Johnson and Deron, Lowry and DeRozen. You have Duncan for however many years he has left and outside him, Howard and Hibbert. Do you really want to build your team around stopping Howard and Hibbert or do you want to spend your money finding the next Paul George/Kawhi Leonard that can play both ways and put the lock down on the other team's best wing? Because players are slashing, a rim protector does have value, but I don't think that Asik has the type of value he would have had ten-twenty years ago when you were gearing up for Shaq, Hakeem, Duncan, Robinson, Alonzo, etc.
Plus, teams tend to be wary of knee injuries on seven footers. There just aren't any dominant centers in the league - Howard is as close as they come, but he isn't quite the same as he back with Orlando. And unless the Rockets figure out team defense, I don't think any of the good WC teams are fearing them because they know they will be able to score at will with or without Asik. A team like Miami would just play small and take him out of the game completely. Asik is a very nice player that does a lot of what you would want in a team, but I think his game was better suited to the years when Deke was the man.