The slam would be a much better contract with North as declarer — any lead from the East hand would give away the twelfth trick. South couldn’t see a way to arrange that, so he bid what he hoped he could make.
West found the most effective lead of a diamond. South won with dummy’s ace and led a low spade from the dummy. This might tempt East to rise with the king if he didn’t also hold the jack. No luck there, so declarer ruffed, crossed his fingers, and ran off seven more club tricks. That left this four-card ending:
South led a heart to dummy’s ace and East had to make a discard that he didn’t want to make. East chose to part with the jack of diamonds, so declarer exited with the queen of diamonds to East’s king. East was forced to lead a spade into dummy’s ace-queen and South had his slam. Well played!