I think his team overplayed their hand. They were expecting the Sixers to blink, since that's what FOs typically do when given ultimatums by stars and power agents. Only that the Sixers didn't, started docking his salary, and claiming that they were ready to wait out 4 years.
While no one expects it to last 4 years, likely not even the trade deadline, Simmons had already painted himself into a corner, and now has little leverage. Yes, he's an elite talent, but his recent behavior, along with his PO troubles will limit what teams offer for him, which in turn might force Sixers to delay trading him (unlikely, but possible if the offers stay underwhelming).
While I would love to have a player like him, at this point, his salary and issues are too great of a red flag to give too much away. I'm sure most teams are thinking like that, but someone will likely bite the bullet, potentially making the other 28 teams regret.