Side nocks work very well with natural strings and because they are cut through to the wood, hold themselves onto the bow. This is very useful with non-waterproof natural glues that may fail in damp conditions.
However, modern adhesives mean that this isn't really an advantage anymore and fastflite etc. strings hold well on front nocks. Another factor is that most bowyers are more familiar with front nocks and won't have a good of how side nocks work.
Jeremy