I'd say that the reason is chemical. The biological viewpoint is of (functional) consequences, mostly what Wreath has said.
The chemistry: stereochemistry, each part of the DNA molecule has a specific conformation, so that the two nucleotides in a base pair cannot confront each other symmetrically. (Hard to explain with words, better look at some molecular models)
As a consequence, one side of the base pair is wider than the other, has more exposed groups.