ahh reciprocating mass.... that sounds you could be onto something. It also seems that for aliminum blocks they use at least 4 bolt if not 6 bolt mains.
Could you intepret that alloy blocks have a greater tendancy to flex when you have large reciprocating mass??

That as the revs increase the amount of time between successive applications of said mass against the main bearings decreases, and thus reduces time available for the bearings to de-stress, resulting in greater chance of failure?

whew got that all out....

And the wikipedia entry doesent say much, just that its stronger, which we all know.