Found that article by Finn Aagaard "Guns of the Kenya Settlers" in G&A Annual 1986, page 148. Some quotes may shed some light on the "popularity" of the "classic" .318 vs 8x60:

"My father once remarked, "I had the chance to buy Karen Blixen's .318 very cheaply when she left Kenya, but the ammunition for it was so expensive, about twice the price 7x57mm ammo, that I just could not afford it.""

"When I started hunting in the late '40s, the picture was still much the same. My first rifle was a Mauserwerke type B sporter that an uncle had liberated during the Ethiopian campaign, chambered for the 8x60mm cartridge that used a 196-grain round nose or solid bullet at a supposed 2,360 fps. It was my only big-game rifle and bagged a lot of game for me, including one buffalo…"

"Another friend, whose father was a prosperous lawyer, had a.450 double Nitro Express, but I have still never seen a .318 Westley Richards or .333 Jeffery,"

I remember another article by Finn Aagaard describing his rifles, but I have not yet found it.