Why was the truck abandoned in the end?

Not true. The Chinese gave these customers a guarantee to which if they did not make it to the US, they would keep trying to send them back until they got there. It's a really expensive business and they wouldn't do that to their paying customers, that they'll in turn have to either give the money back to or try and send them back again. I recently had the chance to see this film and sit in with a Q&A with John Sayles, at which he touched upon that.

Detective Suarez told the Mexican mafia boss who stole his 15kg of coke. It happened before the raid Suarez was involved in. In return for this information the boss had to convince the Chinese to return Rodney. He probably paid them, so the Chinese sent the whole truck and left it for the rescuers, like you wrote.