I think it is because there is more glandular tissue in the peripheral zone, but will be interested to hear what other RG members answer. I looked the topic up, and this paper discusses the different embryologic origins of the zones, but I don't know of the relevance of this:
Bhavsar, A., & Verma, S. (2014). Anatomic imaging of the prostate. BioMed research international, 2014.
Prostate cancer it's a condition of glandular tissue. This cells are most frequent in the PZ of prostate, then the probability of prostate cancer in the PZ is huge.