They're shopping him because they're rebuilding and he's an asset they can give them value back in a trade.
I just don't understand how people here complain non-stop about the receiver group not being good enough and then scoff at every WR option unless the deal is wildly advantageous for the Steelers.
Kupp is an injury risk, but he's a perfect fit for what this team needs and is cost controlled on a reasonable deal (that the Rams would likely be eating money on). He'd cost far more if he wasn't injury prone, the only reason you might be able to get him for a 3rd in the first place is because of that injury history.