My guess, because Punnett was a scientist not a breeder. Punnett did not set out to make a chicken breed that would be recognized and fashioned after the Leghorn. He was trying to understand color genetics of chickens and chose breeds that were interesting to work with, novel or ones that he knew the genetics of to avoid adding in too many variables.
When setting up an experiment, it is best to have a known set of parameters then only change a single one to be able to have clarity as to what is contributing to the data. Starting with a black barred bird is easier than starting with one that has red enhancers or whatever the many variables are with the barred gold duckwing (or whatever the base is for the crele) especially if you have worked with the barred bird before, know what is going on with its genes, and the barring is crisp and easy to visualize.
There must be a reason behind your question. Why do you think he used the Barred Rock instead of a different breed (do remember he used the Campine for some of his experiments)?