Magic resist runes now +8 and...

Because it’s easier to stack mpen early than Lethality. Rune stats are primarily for the early game, when gold can’t be spared for those stats.

Armor is inherently stronger than MR, especially early on, because it provides protection against auto attacks (key for early trades and punishing CS), minion aggro, jungle clear, and turret shots.

You're only going to take magic damage from enemy champions, and generally only when they spend resources to do so. It's simply less frequent than physical damage.