Actually, a lot of Phrag species do prefer cooler temperatures, especially besseae. They can be grown warmer, but will bloom poorly. However, most hybrids are crosses that have at least one warmer growing parent, so they do well in warmer conditions. They will all bloom better in cooler conditions, as the cool temperatures produce darker pigments.
You might want to check out
www.slipperorchids.info for species specific information about temperatures and habitat... there's too much to list here
As Ever,
Matthew Gore