It's not at all - Paladin mitigation is fine. DRKs have MORE magical mitigation but the reason they're preferred is purely because they do more DPS. If Paladins did more dps than DRKs (not that they should) you'd see a lot more of them in the first clears of A4S. Paladins have "enough" mitigation for any magic based encounter, but they don't bring anything at all that the other tanks dont for these, so are almost always strictly a downgrade right now (especially at the start of content when undergeared).
