You balance the encounter around what is there. 10s are easier to skew in your favor if you want progression because you can arrange things so you will enter with a better starting gear level earlier on, a higher personal skill level, arrange to have all the buffs covered and all CDs available and so on. And as a result, players who go that route will have an easier time. Why? Because these players aren't after a challenge. They are after the goodies. And Blizzard cannot really balance an encounter around such players because, by definition, they are exceptional and are taking advantage of an aspect that exists from simply having two size formats.