Board Thread:Questions and Answers/@comment-1754938-20150922055002/@comment-1754938-20150922124845

Smoking gun wrote: I don't know if it was said how long Isshin was captain of 10th division, but I recall in Turn Back The Pendulum it was talked about how captains change often, so it's possible he wasn't on that position long. Rukia was also known as someone who didn't socialize much, until she met Ichigo and his friends. It's quite possible they simply missed any chances of meeting.

Or we can simply chalk it up to Kubo accidentally making a plot hole.

Plot-hole then ! :P