Who hurt you? 
There are various reasons, either
1) the players don't have a lot of time to play UR and it is faster to do missions in Training, or
2) it is because they want to relax, do their missions and not have to worry about winning, or
3) they want to level up some of their characters while at the same time getting some mission progress 
Usually it is a combination of these reasons, for example I am mostly 2), but sometimes also 1) and/or 3)