Probably because, except for biased Celtics fans, nationally and maybe worldwide, he is almost never mentioned as the best NBA player ever. Google it. MJ is almost universally listed as the best ever. In most of those lists, Russell isn't even top 3.
Bill Russell! # 6! The real G.O.A.T.!
11 rings in 13 years!
Plus NCAA and Olympics.
Most competitive ever!
No one else achieved even close to what he did.
I can't even see a reasonable argument for anyone else. How is he not even in the poll???
It's media bias that excludes Bill Russell. His accomplishments tower over any other NBA player, and he did it in an era where he had to battle racism and make historical precedence as the first black coach on top of it. Players in his era never got anywhere near the marketing and media hype that players got in Bird's era, or Jordan's era, which also pale in comparison to the hype players get today.
If Jordan had done what he did in Bill Russell's era, he wouldn't even be in the discussion for these kind of lists.
Russell was the leader of the winningest team ever, but it is a team sport. When looking at the greatest players as individuals, a lot of the team component is taken out of it, and Bill quite simply doesn't measure up individually. He wasn't even the greatest individual player of his era, which was pretty clearly Wilt. I mean Bill joined a Celtics team that had 3 HOFers in their 20's. He never played with less than 3 other HOFers and often had 5 or 6 others. Bill was a tremendous leader and player, but he was always on super-stacked teams. I think it is a fair point to ask, if Wilt and Bill had swapped teams, how each's career would have turned out. I mean when Wilt finally had 2 other HOFers in their prime, he ended up leading the greatest team in NBA history to that point (and which lasted for like 25 years as the best team ever).
EDIT: When ESPN did their list a few years back they had Bill at 7 behind MJ, Kareem, Lebron, Magic, Wilt, and Bird. I would imagine, other than Lebron perhaps moving up, that the top 7 would look the same today as it did when they did that back in 2016, especially since Duncan (who was 8th) probably didn't do anything to warrant moving up more and no one else had come close to cracking the top 10 since then (Shaq and Hakeem rounded out the top 10).