In Gray Tide in the East: Tidal Effects (I forget which book) Hitler still winds up in radical politics, supporting Magyar nationalists in a surviving Austro-Hungarian Empire. I can't remember all the details, but it was a good story.
Not enough time, I would guess. The mound builders and the like were well on their way towards building classical 'empires', but Europeans got into the mix to early and it all went to hell. I suspect that if an ASB decided to destroy Europe and Asia some time in the 1400's, American...
I've always since Conroy as kind of a "pulpy" Alt-history writer. Sure, his ideas are less than plausible, but there's still action and excitement and all that - they're good for a quick read or long plane-ride.