I think that the answer to this one is simple: trajectory!
(while delivering medium game killing power)
At a time when laser range finders or even practicable optical coincidence range finders, and reliable external BDC (bullet drop compensator) turrets did not exist -- outside the military -- I believe that the .270 Win offered something rather unique: the ability to deliver medium game killing power out to 330 yards using the redoubtably simple maximum point-blank range (MPBR) method with a 3 inches high zero at 100 yards.
I do not know about your experience, but in mine, which was 100% mountain hunting in the French Alps, this was huge. I rather suspect that this is what attracted Jack O'Connor to it.
Of course, modern opinion polls are bound to reflect the latest fashionable fad of the day, so I fully expect the various Creedmoor, PRC, ARC, SAUM, etc. to figure prominently, but the .270 Win also beat them in another dimension: the test of time. It is my belief that the .270 Win will still be commercially loaded long after the various Creedmoor, PRC, ARC, SAUM, etc. fade into obscurity...
A nearly perfect balance of power, trajectory, recoil, universal usability, and simplicity
As to Africa, 40 years after selecting the .270 Win as a self-evident choice for my first, long and hard earned hunting rifle, I had to come again to the conclusion, after visiting many, many other options, that the .270 Win remains the near ideal caliber choice for my wife's PG rifle, with an additional edge thanks to modern bullet technologies, that where I had to move from 130 gr to 150 gr (both Nosler Partition) when going from Chamois (mountain goat) to Red Stag (elk), she does it all with the 130 gr TTSX, from Impala to Black Wildebeest. The .270 Win has the killing power, the flat trajectory, the low recoil, the quasi universal usability on PG and MG (mountain game) to make things simple and predictable.
As it did for me in the Alps, the .270 Win allows my wife to simply put the crosshair on it, from 0 to 300, and just pull the trigger. To someone not versed into technicalities, this is huge.
And thanks to modern bullet technology, she does not even need to change load and resight between light and heavy PG.
Certainly, the various Creedmoor, PRC, ARC, SAUM, etc. can do the same, although they are rarely used with the MPBR method, but good luck finding ammo for them now in Africa or at the country gun store... and better luck finding ammo for them at all in 5 or 10 years...