>> What is (are) the limiting factor?  Is it simply that they can't put on
>> enough stores for overwintering, or what?

I've explained this as clearly as I can, both here and on my website and 
written reams on the subject, plus chronicled my experience with splits.

There is no one factor.  It is uncertainty that limits how aggressively an
experienced and prudent beekeeper will spit.

I don't think I can explain any more clearly than I have without repeating 
myself.

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