Nicknamed 'The Don', why is Sir Donald George Bradman considered to be one of the greatest sportspersons in recent history?

Because of the phenomenal number of runs he accumulated as a batsman in the 1930s and 40s in particular. His batting average of 99.94 is far above that of any other player before or since. He retired from cricket in 1948 but is still spoken of in the most respectful of terms, especially in India. If you are skillful enough to get within a bull's roar of any of his records that puts you in very august company.
https://en.wikipedia.org/wiki/Don_Bradman
PS His grand-daughter, Greta, is a clinical psychologist and also presents classical music on FM radio in Oz, so the legend continues.