Because Christmas decorations basically fall into two categories:
1. 'Traditional' (read: idealised Dickensian Whimsy from the end of A Christmas Carol), which tries to capture what people think an old fashioned christmas was like with 19th century stylings because that's what decorations used to look like.
2. 'Modern' which tends to be updated versions of traditional designs or more abstract pieces in glass or plastic. With the advent of pine trees which don't drop needles and artificial trees which can be stored in the attic, this style is even more restricted commercially than you might think as people in very modern homes can, and often do, have traditional style christmas decorations for the short period around the date, even if they otherwise go in for very modern designs.