I have my own theory about this. It's not only people's names that are shortened. The English language does not like long words; more than two syllables and they will often be shortened.
brolly, pub, tv, uni, spagbol, pram, sci-fi, mod cons
there are many, many more.
In other languages, e.g. Scandinavian and German this shortening does not happen to the same extent. Words in these languages are often joined to create very long ones and it doesn't seem to worry the speakers. We like to chop words up. Perhaps they become more digestible that way!
As I said, just my theory.