Ever since the Denver Broncos lost star linebacker Von Miller to a leg injury a mere few days ago, they have been looking high and low for a replacement since the season starts...... today. Free agent Clay Matthews is one of the top choices for a last-minute option, but it looks like that won't be happening.
This is probably the reason why Matthews hasn't been picked up yet. He didn't play poorly last season until he landed on injured reserve with a broken jaw.
The Los Angeles Rams released Matthews early this offseason, which followed with him filing a grievance with the team. Hence the reason another team hasn't signed him; if he takes money somewhere else, that means the Rams don't owe him. I hope that makes sense.
updated with response from agent:
While, above, I assume that he doesn't want to play this year because of said grievance, his agent released a statement explaining that Matthews is still open to playing this season, as long as it's for the "right" team. Aka, if the money is good enough to offset the grievance.