Before runes were turned into the abomination we see them as today, the defensive stats were higher than they are currently. If you took armor yellows and MR seals, which was a staple in a huge majority of situations, you would have 9 armor and 12 MR. Now, if you take the armor and MR defensive stat runes, you get 5 and 6, respectively.
This doesn't seem so bad, except that we have access to a lot more damage and penetration now. With things like keystones like dark harvest and electrocute, coupled with a ton of armor/magic penetration items, the 5 armor and 6 MR is not only a joke, it's almost worthless. Even in the early game, especially considering the fact that you can get 12 AD or 20 AP at level 1 with adaptive force.