Only four horses in the Cup's history have won back-to-back, with champion Makybe Diva the most recent with her three-straight successes from 2003 to 2005.
The Charlie Appleby-trained Cross Counter was striving to become the first international to win the Cup more than once but was forced to settle further back in the 24-horse field than Buick planned after jumping away slowly.
He came from worse than midfield, carrying top weight of 57.5kg, to be beaten just over 1-1/2-lengths.
"He ran a huge race," Buick said.
"He missed the break from drawing barrier five. I tried to get a better position than what we were in but obviously it was difficult so I couldn't get to where I wanted to.
"He finished off his race really well.
"He didn't get the run that he needed."