Played 71% of his games against one team, the remaining 29% were against three teams and all were home series.
Played in only two countries, both known for relatively same pitches. It might swing more in England, but you have to ask whether the balls manufactured then were good enough to swing for more than couple of overs.
Played in an era where video analysis was unheard of, maybe even impossible (I am a bit short on electronics history here). So he could have played with serious technical incompetencies which were never explored by his single major opposition. The only time when they did test him, during the bodyline series, his avg was in the 50s, which is the 'great' territory and not 'greatest'. In the modern age, oppositions change tactics against batsmen every innings, so one has to wonder how Don would have coped with that.
People say that Bradman and Gavaskar were truly great because they played fast bowlers without helmets. For a great batsman with excellent hand-eye coordination, I would think that adding more weight on the head would be more of a hindrance when it comes to balance. To make it worse, usage of helmets hasn't really changed how a batsman approaches the bouncer. Maybe some lesser batsmen are now attempting to hook more. But great players are still leaving the bouncers alone most of the time, with a rare hook played only when they are fully confident of connecting.
The opposition batsmen of Bradman's era include Hammond, Hobbs, Sutcliffe and Duleepsinhji; all four considered amongst the greats. This makes me more concerned about the bowling quality and pitches of that era. My guess is that we should just consider them as good as a Tendulkar against Hayden, Ponting, Waugh and Gilchrist. With the variety of pitches and kind of analysis that Tendulkar and Lara have been subjected to, I do not see how Bradman's career can even be compared to theirs.