Why did non-Russian masters allow Capablanca to play "standard" openings against them? Former world champion JR Capablanca was not a particularly good opening player. Yet he was the "champ" because once he survived the opening, he played better middle and endgames than others. And this occurred because he was able to play a handful of openings time and time again.

Late in his career, he had particular difficulties against Russian masters. Not only the stars like Alekhine, Botvinnik, and Keres, but even second-rank Russian players. A major reason was that they played varied and complicated openings, and got the "jump" on Capablanca in this part of the game.

Why didn't non-Russian players do more to take Capablanca out of his familiar opening lines?
Capablanca's non-Russian rivals did not press him hard in the opening because they didn't know how:


  Capa continued to dominate until the unthinkable happened: He lost his world title to Alekhine. A grossly overconfident Capa entered the match unprepared psychologically for the new and improved Alekhine. In the end Capa lost the match because he had never previously been tested to the degree with which Alekhine pressed him.


Source: Capablanca: Move by Move by Cyrus Lakdawala. Emphasis is mine