Well said Johnny.
Belegbeth said:
The real world is diverse but not incoherent. It makes sense.
Does it? Really? Do we all know the reason why everything is the way it is?
No.
(And I'm not picking on you personally Belegbeth. I'm just quoting you to illustrate my point to everyone.)
One thing to keep in mind is that there is no way to explain EVERYTHING that appears in a campaign setting, more so one so diverse as Eberron. That said, who cares? Will your PCs ever need to know every little detail about all of the entire world's history? Is it so terrible to have to make up something that's not in the book?
Every campaign before Eberron shared one thing. It could be described as "It's like D&D, but take away XXX". You are always forced to remove something, and that something might have been very interesting.
Eberron is different because it has a place for everything and a great deal of effort is being put into finding a
logical place for those things. At least, as much as a fantasy setting
can be logical. Suspension of disbelief and offering a place to explore are the primary goals and I have no doubt the designers will pull that off.
Diversity is a good thing. A very good thing. But you can't expect a setting to come prepackaged with a back story that will fill in every single little detail regarding how the setting became so diverse. The very idea is ludicrous. It couldn't be done in 5000 pages.
So, you take this very diverse setting and you set the PCs down where you want to start them out. You use the information you've been given to provide the framework for your stories. The DM fills in the missing details as needed and everyone has fun.