What I don't understand about football is marquis players seem to get traded for what would be considered peanuts in the NHL. Imagine if like Patrick Kane got traded for a 1st, a 3rd, and like a B prospect. HF would literally cease to exist. Yet in the NFL you get guys like Antonio Brown traded for what, a 3rd and a 5th round pick? I don't get it.
There are a lot of causes for this.
Sometimes it is a personality issue, which is usually the case with diva WRs like Antionio Brown. A lot of teams simply don't want to take the chance on them, so that narrows the market significantly, and salary and contract demands play a big role.
You also have to trade assets for them AND pay them. Then, if it doesn't work out, you could have a large "dead money" cap hit if you have to cut or trade them. That can kill a team for a season or two, which precludes a team from being able to keep their core intact, fill other holes or add quality depth.
Some teams are also scared off by age, more than any other sport there tends to be a physical falloff, so you want to cut or trade a player one year too soon - as opposed to too late.
Some of these guys are simply bat$h!t crazy. And they can tank a locker room and team if the coach isn't strong enough. They get a small group of players on their side and cause problems all season or just outright quit.
Then there are some of these stars who just cannot stay out of trouble. It would be like if Patrick Kane was punching cab drivers every off season, instead of seemingly learning his lesson.