I could be a mile out here, as I'm not aircrew, but have been called on to write in the Tech.Log when an aircraft is in base maintenance - is it possible that the crew prefer to note findings during the flight, then summarise their findings in the Tech.Log once on the ground? Also, it's easier to write in the Log when you're not bumping around in the sky, as they have multi-sheet pages.
I'm more than willing to be corrected on this, naturally - as I said, I'm not aircrew.