Well, the Sixties/Seventies were kind of a perfect storm for base running:
-- construction of a whole generation of new stadiums with much farther fence distances and artificial turf which magnified speed (on the base paths and of batted balls on turf)
-- the competitive balance of the game had swung wildly in favor of pitching and defense. Offensive numbers were at historic lows, thus stolen bases became comparatively more valuable
-- the first generations of nearly-full integration were still working through the game. Guys like Lou Brock and Maury Wills had skill packages uniquely suited to base stealing. Later versions such as Tim Raines, Willie Wilson and Willie McGee would count too. Rickey Henderson was the ultimate of this type.
--Analytics were also working their way through the game regarding the running game. In the Seventies, coming out of a period of 30-40 years bookended by Babe Ruth and Willie Mays, base stealing seemed like a hip, contrarian and avante garde approach to the game. It was the old timers from the eras of heavy slugging in the 20s-40s who worried about throwing away outs on the basepaths.
In the Sixties and early Seventies there were seemingly intelligent baseball people who argued seriously that batting average and stolen bases were just as important as home runs.
With the construction of the urban retro parks with shorter fences and natural grass, power became important again. Corn fed diets helped as well.
Bottom line, Rickey came along at a particular time a superhuman base stealer would be allowed to flourish -- and even then he was accused, sometimes accurately, of playing for his own personal stats.