Omar said at LA Beatdown that Team ICO doesn't want their characters involved with PSASBR in order to keep their "artistic vibe" or something along those lines
I believe this is the answer you're looking for. I'd honestly prefer a statement from Team ICO themselves, and probably a more detailed explaination for why they feel including Wander in PSABR would hurt their 'artistic vibe'. I'm sure someone at Team ICO is probably thinking that it's a great idea because of how dumb and insane it is. The entire concept of Smash Bros and PSABR is just stupid in itself, why not have fun with it. There's a sci fi space nazi fighting a rapping dog, a cat that desires to be a cat, a rapping dog, a thieving talking racoon, a walking zombie warrior, a god, and a demon hunter.
The thing that you guys don't realise is that they don't have to explain themselves. They don't owe anyone an explanation as to why they declined to let someone use their IP. If some random person asked if they could keep your dog for 6 months, would you feel obligated to tell them why they couldn't?