It has to do with the EXP earned, and spacing out the content so you could actually level to 50 while doing it.

If instead of say 7 fetch quests, they gave you 1 fight, and you got a shite ton of exp, and got a full level
or more, it may be a little unbalanced for what the story is supposed to not only do, but also how it should
occupy your time.

In expansions I doubt this will be as much of a problem, as they will expect us to have already have the main
scenario for leveling purposes.