Two reasons: Bragging and control. The Watchtower likes to brag about how many hours are reported worldwide, they feel the preaching sets them apart and proves they are doing God's will. Not mentioned is the fact that the hours are greatly inflated and mostly spent in time filling activities (driving around, donut breaks). For all the millions of hours reported, most people could not tell you what message they are supposedly preaching about, and that is only in areas where they do preach. Most people in China and Islamic countries have probably never even heard of JWs. As a control it's useful to the elders to know who are active and who may be straying from the fold.
All this is in contradiction to the scripture that says to not practice your righteousness in front of men, and to not be bragging. And having the elders bug you about your service report is not good shepherding and is not loving or kind. The bible don't mention a specific time requirement, does not mention field service reports, or pioneering. It doesn't classify people as active or inactive or irregular.