Credit rating is dinged: In the eyes of the lenders that debt trapped you. Why would you still want to be rated well by them?
Country cannot secure international finance: Continuing to secure financing from the people trying to debt trap you is a good idea?
Reputation damage: Same idea, why do you want to keep your reputation up with debt trappers?
Trade sanctions among financiers allies: This one actually has teeth. Don't pay your debts, don't get to trade with your debtors until you do. Hence why Iran and Cuba are locked out of trade with western countries due to their nationalization of western capital.
Taking on debt as a precondition to accessing a trading bloc so they get you hooked onto trade and can use that as leverage to force you to pay up is a pretty good power play. Of course, that leverage decreases if have other trade blocs as options.