Oddly, and when I was in and out of Frank's Drum Shop every weekend in the 60's, I never much took to Slingerland Drums.
Seemed like they were the Red Headed Step Sister of the lot. Maybe Maurie Lishon didn't properly showcase and display the Brand much.
As a businessman at that point in time, perhaps his foremost desire, and like many other dealers in business, was to sell what sells. ($$$$)
Maybe when rock and roll really took off, Slingerland's advertising was aimed at the "old jazz geezers", and a lot of drummers took alternate paths and unwittingly actually bought some inferior Drums in the process. I know I did as well back then.
Even Rogers seemed a bit out of touch.
I'd garner a guess, there weren't too many who had their hearts set on a brand such as Sonor. But Sonor has made some really great drums.