Because in my experience as a Greek Orthodox, the definite majority of parish priests are married (at least in the Chicago area), although there certainly are a few single ones.  The official Greek Orthodox rule is that Priests can get married only before they are ordained (many seminary graduates actually put off ordination for a few years so that they can get married before they're ordained).  However, in order to become a bishop, you need to be unmarried (I think widowers can become bishops as well).  So there is a bit of a trade-off between either being celibate or giving up any chance of being a bishop, and it is also true that the large majority of married priests become parish priests while most single priests go to monasteries or teach at seminaries.  So while it is not "officially" necessary to be married to be a parish priest, it does appear to help your chances of getting such an assignment.