Probably because it usually doesn't involve any commercial, that is, monetary, transaction, and it doesn't involve any negotiation; it's just slapped on by someone you've never talked to. How would it be considered a commercially negotiated contract?
A contract doesn't need to have any money change hands, as long as both sides get something out of the deal (have "consideration"). The GPL does have this as one side gets the right to use source code and the other side gets guarantees on how that source code will be used.
Also, being slapped on is not a problem for a contract. We interact with adhesion contracts every day that are slapped on to things. When you accept a valet ticket for parking it has a contract on the back that you are assenting to by using the service. No negotiation occurs and adhesion contracts are valid contracts.
Since "commercially negotiated" is not a term of art, why do we think the GPL is not one?
I'm not saying the GPL is an invalid contract; I agree that it is a valid contract. I gave reasons why it may not be considered "commercially negotiated". If they just meant any valid contract, they would've just said "contract" instead of "commercially negotiated contract".