Why are there champion trains in low-level maps?

I recently started playing GW2 again and noticed that there are champion trains in low-level maps like Queensdale. A champion train seems to be a semi-organized group of people killing the champions in a given rotation. Those trains will cycle the rotation many hours.

However, shouldn't diminished returns make such behaviour less profitable? Or is there something new that you can get from champions only (haven't played since May).

Solution 1:

Apparently the champions are being farmed for their loot bags, which have been added in August. The Queensdale rotation seems to be one of the most-efficient, since the champions are quite easy and the map also provides the Shadow Behemoth for a additional bonus.

Solution 2:

This is also a quick solution to complete your monthly "Champion slayer" achievement. As it is the case in October 2013.