History tells us that Christianity has evolved considerably over the past 2000 years, and what it is today is far-far different than what It was before.
The way it is practiced certainly has (evolved), but not its core principles...but that's entirely beside my point.
Your original question was, if I understand correctly, why Christianity didn't appear sooner than it did. My response was simply that if it had, it most likely would not have had the human infrastructure to propogate and perpetuate itself. We have other religions that pre-dated Christianity which have died out entirely or thrive only in very specific and limited areas. We have the Roman cults which were, generally speaking, entirely overwhelmed by Christianity, and the empire they served became the western culture, or the springboard from which Christianity ultimately spread about the globe.
While I certainly believe that God is omnipotent, He is certainly not beyond asking humans to do a little work on His behalf. Asking why Christianity took so long to appear is, IMHO, like asking why a chef doesn't serve the meal before he's acquired all the ingredients.