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.

A bell curve of test cricket averages shows Sir Donald Bradman is six standard deviations away from the mean. He was a statistical outlier. See graphs on these two webpage:
https://www.reddit.com/r/Cricket/comments/423eby/test_batting_averages_bradman_is_over_six
https://significancemagazine.com/did-don-bradman-s-cricketing-genius-make-him-a-statistical-outlier/

Apparently, a good batting average for a modern-day batsman is 40+ with those above 50 being exceptional. Bradman's average was 99.94. The second highest average of all time is 60.97. [now 61.87]

https://news.ycombinator.com/item?id=34043485

List of highest test averages: https://www.espncricinfo.com/records/highest-career-batting-average-282910

Additionally, opposing teams had to introduce controversial, intimidating, and dangerous tactics to try to curb the Don's scoring ability.

The controversy reached its peak during the Third Test at Adelaide. [...] In the third over of the Australian innings, Larwood bowled to Woodfull. The fifth ball narrowly missed Woodfull's head and the final ball, delivered short on the line of middle stump, struck Woodfull over the heart. The batsman dropped his bat and staggered away holding his chest,[...] [England's Captain, Douglas] Jardine called to Larwood: "Well bowled, Harold!" Although the comment was aimed at unnerving Bradman, who was also batting at the time...

Consequently, new rules were introduced which changed the way Test Cricket has been played ever since. https://en.wikipedia.org/wiki/Bodyline#Changes_to_the_laws_of_cricket