Hmmm... the OP asked a question and received a very plausible answer in the first response.
Unless you were involved in drafting either set of rules... it's just speculation on your part, and your speculation is just as valid as the next person's. Nothing to get worked up about. The earliest rule book I have digitized (1989), specifies that #4 shot is the largest allowable. Somewhere I have rule books from EOT in 1986 & the original SASS rule book from 1987. As I recall, both specify the same. So that rule pre-dates my earliest recollections in cowboy action. If that rule follows the trend of other rules, it's in place because someone use a larger shot size and created a real or perceived safety hazard. 'Nough said. I've used #7-½, 8 & 9 shot in both CAS and WB and haven't encountered a difference worth noting.
The earliest WB Handbook I have digitized is from 2009, the first edition and specifies the largest size shot as #6. No explanation provided. GJ's speculation is certainly plausible. Maybe PM Happy Jack, as I seem to recall his being involved in drafting the initial WB rules. Or, at least an early member of the WBROC.
Really? Differences of opinion are inevitable. Expressing varied opinions are what make us THINK. It's called communicating. And communicating is what makes a community. SASS is a big community... I don't know anyone that agrees with EVERYTHING... And if they do... I have to wonder they've ever had an independent thought?