It has to do with the size of each molecule. One is large (double ring) and the other is small (single ring). If 2 purines would pair, pyrimidines next to them wouldn't be able to bind with each other, and if two pyrimidines would bond, there wouldn't be enough space for 2 purines next to them to fit. By having them pair with each other, the spacing along the length of DNA or RNA is kept uniform.
