Why has pastor been the most commonly used term for the office over the years?

1. Why has pastor been the most commonly used term for the office over the years? 

I’m not entirely sure but it may be because it captures the essence of the task. Pastor is simply the Latin word for shepherd as mentioned previously and it seems to me that shepherd most succinctly and most richly describes the Christ-given assignment from the Chief Shepherd and Overseer of our souls.