I think people began to love the strings many years ago even since Christmas was founded. Maybe they wanna make a remarkable sign for Santa Claus. 
The earlier ones are assembled by the bulb in my memory, and LED instead of the bulbs, to save the more electric and safer working situation. 
The most important reason is the amazing light effect from those string lights, I think. Everything becomes warm and lovely when they covered by a string.

There are so many kinds of light for Christmas decoration, it seems like a match among the houses by these beautiful lights.