It's because he was so far ahead of his peers and did it in an era where pitches were like minefields. He was averaging around double than his peers during the time. In an all time context, averaging 100 runs per game is pretty much unheard of, great players normally finish on around 50-60 so you can see how far ahead he is. The fact that no one else has a similar record during that period shows it wasn't an easy thing to do. And as mentioned above, the pitches were very difficult to bat on (uncovered pitches, less stringent rules on bowlers ) so his technique had to be pretty much water tight, cricket historians often pointed out his technique was text book, so he wouldn't be behind in terms of the technical aspect of the game.
Wrt to how he would do in the modern age, most cricket experts would say he would do even better as batting has become easier (pitches, modern bats, rules generally favouring the batsmen), it's absurd to think he would average more than 100,the best players in this era are hovering around the 60 mark. It's quite similar to the Pele debate and how he would do in the modern era (better playing surfaces, footballs, protection from referees etc) .