Actually, the why is kinda obvious. According to the New Testament "On the third day he rose again," referring to the resurrection of Jesus Christ.
What's interesting in D&D is that the resurrection of Christ would not be a resurrection under the D&D rules - it would more likely be the result of a Raise Dead spell.
Which makes me think about the use of Raise Dead and Resurrection (you died a messy death, or one a long long time ago type of spell - from the players' perspective, more likely messy death.)
In game terms, is a Raise Dead spell written as it is because the assumption is that the soul of the deceased hasn't yet reached it's final destination in the afterlife. The soul's attachment to it's mortal body is still strong, thus requiring less power (2 levels lower spell level) than a resurrection spell?
Is Resurrection (on par with a magic-user's Wish spell in power) recalling the soul from it's eternal resting place? If so, what happens if the soul recalled does not want to return?
This is what Easter does to me. Makes me think on multiple levels ;)