The CTS wagon has a higher base price than the SRX crossover. Given that many buyers already prefer crossovers, it's not terribly surprising that the wagon didn't sell well. Even within its own brand it was pushed into an impossibly small niche.
It was also priced higher than the 3-series wagon and A4 Avant. Even though the CTS is a bit larger, I would imagine those two cars stole a lot of its sales as well.