Why does Christmas cactus only flower at Christmas?

The Christmas cactus [Schlumbergera and Zygocactus spp] blooms in response to appropriate heat, light and moisture levels. At the same time, it blooms in response to its genetic inheritance. It's a native of the Brazilian rain forest, where its growing season corresponds to northern winters. The appropriate environmental conditions, the genetic inheritance, and the growing season combine to make the Christmas cactus bloom when it does.