Those are completely different animals living in totally different niches. Triceratops weighed more than T. rex for a start and is far more agile than a walrus on land.

Also there are Rex skeletons that show gouging in the bones from triceratops horns.

Much like lions often die trying to kill buffalo or elephants, rex's would have been taking a huge gamble hunting triceratops. That's not to say Triceratops also didn't also die regularly to T. Rex.