How many famous African hunters can you all think of who used a .30-06 Springfield to take members of the African Big 5 ?
Two that come in mind immediately: Theodor Roosevelt, and Ernest Hemmingway.
From various writings, I came to conclusion that Hemingway did not overly like his 577 because of recoil. Besides, his first safari was done without 577, and on the other hand he just loved 30-06. To be noted that his favorite calibers were 22lr, and shotguns (he was shotgunner first, then the rest).
TR rifle for safari was new design (1903), and only three years later US Army switched to different bullet (1906), he went on safari 1909 - so his caliber could be 30-03 or 30-06. (practical twins)
30-06 on DG in general: I think this has to be looked at in the contest of time, as a step in evolution of hunting DG from black powder to modern standard.
30-06 was then new, seamed like "magic caliber" of the time when majority of rifles were still black powder.
There was lack of caliber regulations or restrictions in many countries.
Famous hunters were celebrities of their time, their equipment was know, etc.
In the same time... I am sure much more poaching was done by whatever was at hand, by thousands of poachers in last 100 years.